It depends on which hemisphere you're in. In North America it's traditional to wipe from the center out, while 'down under'* its traditional to simply rub up against a tree and hope for the best.

Certain advances in nanotechnology have reduced (but not eliminated) the need for both techniques, but you may find a combination of the two is the still the best way to 'cover your bases'. The important thing is that you wipe early and wipe often.

*Australia, New Zealand, Europe and the rest of the world