Squish- this is straight from my vet's mouth. And I love my vet..

It is better to put her down now (my dog had her throat ripped open, on top of heart worms, a bowel problem, erlichia, and no strenght left to fight) then to wait and make her suffer through the tests it would take to discover what exactly we can do to help her.
So, if you are still on the fence, pick 3 things that are your dogs most favorite things in the entire world. And when she can't do those anymore, it is time. My girl's were digging, climing fences, and eating. She quit digging and she couldn't climb the fences anymore and the last time I fed her, she was less than enthusiastic. It was time, and I still wonder "what if".
You might remember that we actually shot the last two we put down. They were beyond help, both with cancer, both dying slowly and painfully. We still wonder if we did the right thing. They both went peacefully, didn't even jerk or spasam. It was better that way. I know it was. It doesn't make it any easier though. And if you want, I can bitch slap Jason. He is being a prick and I agree that he is just using it against you.
If it makes you feel any better, we had my first girl euthed. The first part of the injection, she layed down. Then she closed her eyes and quit breathing. No jerking, spasams, etc. It was quick and peaceful and we didn't have her sedated first. Hope that makes you feel the tinyest bit better.