Good question. When you are on, you will find that you can train harder, maintain more stamina and lift heavier. This all equals more muscle damage in the gym. As long as you are on a good training program and working hard, there is no need to increase volume just because you are "on". Remember that tendons, connective tissue and joints all are going to need rest too. Doubling up, while already working harder, may equal disaster!
I'm not saying that there is never a time to increase volume. Sometimes more volume is what is needed for a specific muscle group, in a specific situation.
When you go into PCT, you want to still train hard and lift heavy, but you may want to slightly decrease the number of work sets a little. Get in there, lift heavy and then get out.