How do you normally determine when to end your set assuming your goal is to maximally challenge a particular muscle or group of muscles?
A) When you hit an arbitrary predetermined number of reps
B) When it starts getting tough
C) Several reps after your form has completely broken down and the goal of the exercise is unrecognizable
D) When you can no longer voluntarily control the exercise with the intended "stuff" (maintaining joint positions and the plane of motion)
While the answer may seem obvious, it's the least common reason of the four. If you keep repeating the same tradition by following the crowd you will get no further than the crowd.