When there's any traffic around, I signal my turns with my left hand. When I'm initiating a turn, that means I'm riding one handed while simultaneously braking (usually, as I live and ride on hills and I'm so gosh-darned fast ). I want the hand on the brake to be operating the rear brake. Less chance of the front wheel going out from under me while I'm making the turn.

I'm left handed, but that doesn't really factor in, here.