I was actually working on an article on this exact subject. Unfortunately after 3 drafts I just couldn't get it down on paper the way I wanted.
I believe that people have things backwards. Most people are obsessed with good form on curls and isolation movements, but don't put nearly the same effort into performing compound lifts with great form.
In training, big lifts should be done almost perfectly all the time. But if you find that you can't prgress on things like rows, curls, and OHP's without a little help from some extra movement, you should do what it takes to get it done until you reach the point where the muscles you're trying to train aren't doing most of the work anymore. That's the time to drop back down and start progressing back up again.
I agree overall, however. Sometimes to place maximum tension on a muscle you need to include some cheating in your form.
Raw gym PR's, soon to be broken-
Squat- 465 (405x3)
Bench- 345 (315x2)