Depends on the road. Often there is plenty of shoulder and if so I will use it. Otherwise there isn't much of a "fog line". If the road is wide I will stay relatively close to the edge, if it is narrow I will stay closer to the center. Also if the road has a steady flow of traffic I will stay closer to the edge and if it is less traveled like many of the mountain roads I will stay closer to the center. Actually for the mountain roads it is usually more of a concern that another cyclist descending will coming screaming around the corner. If I am descending, I always take the whole lane, but may ocassionally slow and let a car by if it is clear to do so.
I suspect your location may help to reduce the effectiveness of a mirror...it sounds like your rural roads are wider than up north, and also because you have a strong cycling culture there in Santa Barbara. I think the more of us there are on the roads, the better we're treated by traffic.