The piston will be faster in the direction that the piston taper is. So if the piston is facing up towards the shock cap, compression of the shock will happen slightly faster. If the Piston taper is facing the bottom of the shock then the rebound of the shock will happen faster.

This is what I have read on the subject. If I'm wrong someone please chim in. I have always run my tapered pistons down to increase the rebound speed. If the information I was given is correct then to me it makes sense because in my opinion you want your tire to spend as much time on the ground as possible so having it rebound faster to me means just that.

If Chicky can chim in here that would be great cause I've noticed he's been running his pistons up according to some of his setups.