.
I mount it non-drive side. I'm in the U.S., and although I'm often controlling the lane, sometimes I'm riding at the edge of the lane to let cars pass, or if there is actually a big enough shoulder or a wonderful doorzone bike lane. Since the road should have some camber to it, if I mount the pannier on the drive side, I've got to work harder to counteract how much it is pulling if it's heavy and it often is.
This also makes me look a little wider to overtaking cars, hopefully giving me a little more room when they try and cut it too close, and puts the reflective elements of the bag on the side where it is more useful most of the time.