I use both hands...on both sides. There are some areas of my face and neck, and some strokes that are easier to reach/execute with one hand or the other.

I'm still figuring it all out, so I don't shave the same way every time, anyway. And it depends on the blade, too; I switch hands differently depending on the blade's width.

Proinsias said it well: being able to switch hands gives you more options.