Depends on the road. Multi lanes, i.e. Interstate I'll ride left of center in the left lane, gives me more road to work with when the idiot in the middle wants to move over. Two lane road, normally right of center. If I'm in a position such as where a right merge lane is coming in I'll ride right of center to ensure cars merging can see me better.
I rarely ride dead center, especially at a stop, I barely reach the ground (5'10"/Ultra Classic), if I'm on center both sides of the road slope down and away, if I'm in the 'troough' of a tire track both sides slope up, gives me some extra ground for the toes to catch.