Now this is interesting to me. How bad do you have to feel before you begin to make these judgements? I am still relatively new to this whole weightlifting world, so I am not sure about my own reactions to training.
I know some days I feel like hammered shit, I can't even do my normal activities without a serious hitch in my giddyup, and there's no chance I can workout, but I go anyway and I manage to do some decent things, even if they are less weight than the week before. I also know for certain that some days I wake up stiff as a board and truly wonder if I'm going to make it through my session at all, much less my day, but then start to feel decent once I start lifting and get going. Then other days, I feel pretty sore, but fairly decent overall, and yet when I start to workout, I can't really do anything anywhere close to decent weight.
So, what's the line? How much soreness is acceptable? Or is it all mental? If your brain isn't gung ho, then you call it and move on?