Some authors claim that working till failure is critical (for example: Mike Mentzer).
What is failure? When you get to a point where you can't complete another full rep on your own, that's failure.

Probability of myofibril’s micro injuries[^1] increases as the level of creatine in muscles decreases over (working) time. But, over working time ATP’s recreation speed (using Anaerobic Alactic&Lactic Energy Systems) increases and thus the probability decreases. Therefore the longer set duration, the less effective (cause less micro injuries) last repetitions. Thus work after failure (for example: drop set) has little meaning, unless failure was reached within first 10 seconds.

Training till failure isn’t your goal, but as you become stronger and intensity increases you’ll start reaching failure. A word of caution: don’t train till failure in “wounding exercises” (“wounding exercises” are exercises in which bad execution technique has higher potential of causing injuries), like: squat or dead lift.

[^1]For more information see: muscle damage theory.