If you choose to decide to cheat in order to complete a set, be sure to do so only during the tail end of the last set of an exercise. In other words, cheat only when you're very close to muscle failure. Muscle failure occurs at the point during a set when you would not be able to complete another repetition by yourself even if your life depended on it. Then, and only then, is it permissible to cheat when performing an exercise. The purpose of cheating in this case is to take a muscle slightly past the point of muscle failure and into the realm of near total muscle fatigue. Maximum muscle fiber destruction occurs at this point in which the goal is to gain more strength and muscle mass.
Brian Danley, CFT
"Motivation is what gets you started. Habit is what keeps you going."