You don't have to do anything but provide for basic needs of the plant. Topping is just an optional technique some people use where you cut the leader node of a seedling/clone after a certain point in growth to increase side-growth. Basically you get several smaller colas as opposed to one big one. You don't need to do this.
If you use 'miracle' grow on your plants, yes, you're going to want to flush the shit out of those mother fuckers at least before you cut. Or feed super super lightly... and what about bloom anyways? You don't have to do this in living soil but chemical salts, it's a good idea.