I underbore every time. Last CPPS, I was using a .681. I don't think I've used anything below a .685 for a loooong while.
My logic (it may be flawed) is that since paintballs are slightly compressible, forcing them through a smaller barrel will create a nice seal and allow all of the air to push it instead of some escaping around the sides.
Then again, one of my mates overbores to hell for the same reasoning. It's all semantics - if you can't hit them with your paint from far away, get closer and then shoot them!