Pavel Tsouline believes that you should never ever go to failure in ANY set at any time. He believes (if I remember correctly) that it will train the body and mind to fail at that exercise, set, rep scheme- to fail every time.
I tend to follow his approach a majority of the time, but every now and again (maybe every 4th workout per part) I'll go to failure on whatever exercise I can. I like to leave a rep in the tank, for the most part, and once it gets hard and I'm not so sure I'll get the next rep, I terminate the set.
And no, I'm not Bob C!
I'm not sure if I believe that. What rep you fail at gives you a reference point for you next workout. Say you can do 8 reps at 275 on the bench, failing on the 9th. Then the next time you bench you can now set a goal to surpass your previous performance, in this case, trying to do more than 8 reps. This also provides some feed back as to how your training is going because say in the next workout you only get 7 reps and same with the next workout, i.e., you're not progressing or actually getting weaker, then you know something is wrong -- over training, diet not in check, not getting enough sleep, etc.
Arthur Jones use to say that always working within your functional ability, i.e., performing something that you can already do, will do little or nothing to stimulate an adaptive response. You must, as Jones' put it, attempt the momentary impossible. Say you can do 8 pull ups never attempting a 9th rep, it doesn't send a signal to your body that it has to adapt.
Arthur Jones is not to be taken lightly.