Arthur De Vany's Evolutionary Fitness Art's Blog on Fitness, Health, Aging, Nutrition and Exercise

No harm. Never ever hurt yourself. If you do damage you will become discouraged and quit. Women must protect their rotator cuff area, as they tend to be weak there. Men need to protect their egos. If they try to emulate people they see in the gym they will not see good form normally observed good practice. I particularly did not like exercises that are done to full failure. This is self-defeating because the muscle is already so full of lactic acid as a means of protecting itself that you have little strength left on those last sets you squeeze out. If you focus on the negative part of the move, which you do slowly, and not to full joint rotation, there is little possibility you will hurt yourself. I would add to the no harm principle that if you experience excessive delayed onset soreness one or two days after the workout you've compromised your function for those two days and that loss is far more important than the slight gain you might make from overdoing it.