While in Edinburgh, Scotland last year, I bought hose and flashes at Hector Russell. I asked this very question, and was told that it was a matter of preference. There is no right or wrong.

I was told that men with large calves (like me) tend to wear them to the outside, which I believe you are describing as "9 & 3". Guys with smaller calves wear them in more of the "10 & 2" that you described simply because they don't have as much leg down there.