If there are two lanes in your direction, there's no way you can be impeding traffic significantly. In any case your safety is paramount and if there's not enough room within the lane for vehicles to pass you safely, then logically they have to move at least partially into the other lane to get around you. Given that that is true, then it doesn't really matter where you are in the lane, they have to change lanes anyway. The farthest right I'd ride in these situations is the right tire track. I move farther left until people stop trying to squeak by me.

Sometimes you can't win though; I've been right in the center of the lane and had people go around me with oncoming traffic present on 2 lane roads, pushing the oncoming traffic into the DITCH. Some people feel that it's enough of an imperitive to pass bicycles that if they have to, they'll cause an accident rather than wait for 5 seconds behind a bike.