In town, I tend to stay on the left third. I feel I am more visible there, and it gives me a bit more negotiation room in the event that someone pulls out from my right..
Out on the road, it depends on conditions. On our ride over Lolo pass last year, we were headed east. The river on our right, and typically a rock wall on the left. The wall blocked visibility to oncoming traffic, so in the left hand curves, I started from the right third, but carried just enough speed so that I only used up to the center...making sure my body did not protrude into the oncoming lane.