Brandie sent in a rather lengthy letter.  For the sake of brevity, I've edited it a bit, though I appreciate the detail I got to read. 
When [my lab] was about 8 years old I noticed she was  having trouble keeping up and getting tired a lot. Extensive testing  only revealed mild hip dysplasia.  At about 9 1/2 years old I noticed she was walking differently, more  stiffly, and it would be especially noticeable after she did a large  jump. So another vet visit determined that it was merely arthritis and  hip dysplasia working against her. She was put on pain meds.  She seemed to continue to worsen gradually, which was expected. But  then suddenly she seemed to be worsening more and more rapidly, much  faster than myself or her vet anticipated based on what we thought it  was at the time (arthritis and hip dysplasia). I prepared myself to have to make the decision to put  her to sleep within a couple of years at the rate she was declining.  After a couple of months of rapidly getting worse, she woke up one  day much, much worse than she had been, enough that I wondered if she  had injured herself in the night. I took her to the vet again, who  discovered that it was nerve damage causing this problem, not the  arthritis and dysplasia (although those things certainly didn't help  matters). She said she figured out it was nerve damage because of the  way my lab was dragging her toes when she tried to walk and that she  could bend my labs toes under her pads, place them on the ground, and my  lab did not correct it.
She was unable to determine exactly what the cause of the nerve  damage was, but strongly suspected a tumor in the spine, one that  possibly may have been growing for a couple of years and could have been  the real cause of her slowing down the way she did. She said that I  could take her to the university hospital a couple of hours away and  have a test done to find out if it is a tumor and if anything could be  done, but that the test would be really expensive.   However, she told me that the test that would have to be done would  be very hard on my dog, and might not even discover the cause of the  nerve damage but could worsen it. Additionally even if the cause was  found, the surgery and recovery involved would also be very difficult  for my dog and could take months and would be no guarantee that it would  help enough to have been worth it. She said that typically nerve damage  does not get better, only worse if there's any change at all. So I  decided not to put my dog through the testing for her own sake, since  there was little chance that it would really help her without great cost  to her. 
I asked the vet if she thought that I should put her to sleep. The  vet  was very ambivalent with her answer. I know she was just being  professional but it frustrated me to no end because I was afraid I could  not properly judge my dog's quality of life because of bias. I feared  causing her to suffer and not knowing it.
 
I got a harness for her back end and helped her walk  anywhere she wanted to go, and helped her go to the bathroom for the  next couple of months. She didn't mind having to do things that way. The vet told me that there was a 50/50 chance she could either feel  nothing or be in excruciating pain.  After those couple of months, I noticed she seemed to just be  getting tired, and not just because her front end had to do so much work  to compensate for the back end. However, she was still happy and playful and outgoing as ever.  I  suspected she was started to hurt quite a lot judging by the change in  her panting patterns, the positions she slept in, and how she reacted to  events in life compared to the past. But, I could never really be sure.  As soon as I would decide that she was indeed hurting and was just  being really good natured about it, I would second guess myself. No one  else would really give me advice about it. The vet refused to say one  way or the other. So I had to make the decision completely on my own,  and I decided to err on the side of comfort for my dog. 
I thought long and hard about it and decided that I would rather put  her to sleep too soon, than to wait until she was hurting so badly that  her seemingly unbreakable happy spirit started breaking, or worse yet,  wait until something happened that the only humane choice was to  immediately put her down. So I got one last month with her to say goodbye, and in the week  prior to the big day, all rules were thrown out the window and she got  to eat and do anything she wanted. I spent that entire week making it  all about her and doing everything I could to make every hour as happy  as I could for her.  
Mission accomplished, she was extremely happy, bouncy (as well as  she could be with her back end out of commission), playful, outgoing,  etc right until the very end. You'd have never known she was having the  problems she was having if you didn't know her unless she tried to stand  up and walk. And therein lies the problem for me. I thought that  knowing I would rather do it too soon than too late would keep me from  regretting the decision, but alas, it didn't work that way. I have since  wondered if I did it too soon after all. it has been 10 months and I  still cry wondering if I should have waited and given her more time. I  wonder if I just imagined all those signs that she was in pain, or  imagined the feeling I got from her that she was ready to go if I'd let  her even though she was still happy, and constantly second guess myself.  I wonder how much longer I could have had with her if I had waited  until her happiness began to fade before going ahead with it. What if,  just what if, she would have gotten better?
All I've wanted since that day is an objective,  educated voice to give me peace of mind by telling me I did the right  thing, or be honest by saying I did it too soon and giving me the chance  to process that so I can let it go and work on forgiving myself. Some  people have told me that I did the right thing, but it doesn't help  because I had several people question my decision at the time as well. But it's not like I could really ask my vet if I did  the right thing and expect anything but a professional answer, either.  As frustrating as it can be I totally respect that. 
I am hoping since I am not a client of yours, that perhaps you will  disregard the need for professional neutrality and offer your educated,  experienced, and objective opinion. You would do my heart a world of  good, either way. I think not knowing if I messed up is harder than  knowing I did mess up. 
Okay, so here goes my response, probably equally as long.
First, Brandie, let me say that based on what you shared your vet did everything correctly and appears to have given spot-on information.  Spinal tumors can be very difficult to discover and usually don't show up on regular x-rays.  Combine that with a breed that is prone to joint problems and some signs of arthritis, and anyone would likely come to the conclusion that your vet did.  Your vet was probably talking about a myelogram, where they inject dye around the spinal column and then look at the pattern of that dye on an x-ray or other imaging method.  Yes, neurological damage is hard to predict and usually doesn't get better, so in this circumstance I can certainly understand not pursuing that option.
I often get asked "should I euthanize my pet?"  This is a difficult question for any vet to answer.  Part of it is professionalism.  We don't want to force someone into a decision that they're not ready for, and ethically we shouldn't make decisions for the client.  Our job is to give the client enough knowledge to make an informed decision on their own.  Even if I'm 100% behind the euthanasia, I ethically and legally can't make that decision for any pets other than my own.  The final decision has to come from the owner, as hard as that can be for them.
Another reason why vets can't make that decision is that we only see the pet for a short period in our hospital.  Even if someone is giving great information and descriptions, we can't really tell how their quality of life is at home.  In these circumstances the owner is the only real judge, and we have to rely on their intimate knowledge of their pet.  The only person who can really determine a pet's quality of life is the family that lives with them.  A vet can guide, educate, and give opinions, but because of the emotional bond only the owner can truly determine the elusive quality of "suffering".
Now with all of that being said, there are some things that a vet can do to help the decision.  I have said in particular cases that I don't think euthanasia is necessary.  When I think it's valid, I'll give the owners all options and include euthanasia in that list, letting them know that if they choose it I would agree with them.  I can say that the odds are and whether a given disorder is likely to be painful.  Again, all of this is merely to help the client make the decision on their own.
But you came to me for some closure and assurance, Brandie, and I'll try to help.  Your vet made sound decisions and gave you solid advice.  Your dog likely had an untreatable and eventually fatal disease.  Before she died, she would have gotten progressively worse and have suffered even more.  You allowed her to pass away when she was still in good spirits and wasn't hurting.  Euthanizing her wasn't a matter of "if", but of "when".  If you had waited, you would have been postponing the inevitable and getting to the point where you would have been more heartbroken at her condition.  By making the decision when you did, you were able to do it on your and her terms, with peace and dignity.
You did the right thing.