I would do lower reps than 15-20 if you're trying to gain.I have always read that drop sets are best to bust through a plateau.If you do them all the time you either overtrain,or become used to them,and they wont act as a shocker anymore.
I'll be at work all day,but just in case you say i'm doing higher reps because i'm cutting.Many are saying that's one of the biggest misconceptions today.I would do your basic staright sets of 6-12.You could finish off with a drop set.If you have been doing nothing but drop sets for a while though,i would take a break from them.