When I'm intentionally staging a bike for a picture, I
try to put the cranks at 3 o'clock and position the valve stems according to where I want the tire labels, usually either top center or bottom center. I want to emphasize that I "try" to do this, but I generally fail and don't bother to correct it when I look at the picture and see that I failed, so you get what you get, typically something very slightly, but definitely noticeably, off from what I attempted.
Crank arm at about 3:08, valve stems at 5:53 and 5:46.