The only reasons I don't use primary all the time is because I don't want to be selfish all the time, and because it may encourage someone to hit me deliberately if I carry on for too long. Otherwise I would use it all the time.
For myself, my default position is to ride just to the right of the middle of the lane, around the right tyre track. In the ideal world I would do this 100% of the time.
In reality I modify this position based on a variety of factors. For instance, if I see a vehicle behind me and I'm on a normal sort of two lane road, I move left to the point that it still isn't possible to overtake without using the oncoming lane. This 'shows willing' (although it doesn't actually help the motorist to pass). On a road with steady traffic the implication is that I ride here almost all of the time, which is true.
If there is a solid stream of oncoming traffic such that I'm going to be blocking the car(s) behind for too long (a subjective judgement based on how long I think their patience with being safe and responsible will last) I then look for a place to pull over to let them past. I see this is a good exchange for blocking them when I'm moving.
I think 1:10,000 and 1:10,000,000 are still gross exaggerations BTW. I'm sure that on a long audax something like a thousand vehicles must overtake me (so between a field of 30, that's 30,000 passes). I've never been on an audax where someone was clipped by a car.
