i take the lane on the busy road i commute on (2 lanes each direction) so cars don't really have a problem going around me except in the evening when the traffic is heavier. i try to keep my speed up as best I can (wish my MTB had a lower gear).
but i also take the lane on the back country road with no shoulders, just ditches, it's still pretty heavily trafficked and its hilly so i tire out easily, so i often pull off at a driveway every so often to let the pile of cars behind me pass (assuming they didn't have a chance to go around me).
but i'm always at least in the right tire track, or the middle.