I usually ride in the middle of my lane with no traffic so I can be better seen by approaching traffic. (No shoulders here either) As a car approaches from behind, I move over to the right to allow plenty of room to pass. The exception is when a car slows behind me and appears to be contemplating passing between me and oncoming traffic. Then I will take the lane and as soon as the oncoming traffic passes I move right again. Another time I take the lane is when approaching intersections. There are exceptions to any of this of course. No one rule applies all the time. Most motorists around here are courteous though it amazes me the numbers that will pass me in blind curves fully in the left lane, or move over almost completely in the left lane to pass with oncoming traffic approaching. I've seen the oncoming traffic have to stop or even forced to move off the road. Blows my mind how many can't wait 15 seconds to pass.