First I was very conservative with when to quit sets and wasn't making much progress. Then I read some of Coach's comments and the thread on when to stop sets and was making some fantastic progress.
Then I hit week 9 on a bunch of exercises. It quickly became obvious that it was a too much extra volume but I was excited and for the first time in a while I tried to grind out the last few reps on the last set in 3 different exercises. Now I've got a mild tendonitis in my shoulder and elbow and I'm just waiting around for it to heal and not progressing. At least now I know where the line is between pushing hard and pushing yourself into an injury (and it's giving me some time for my arch body hold to catch up...).
Don't grind.