I like to mount the prop so compression starts at about the one O'clock position. I tend to flip my engines by hand and its just easier to flip across the top and slightly downward.

When I flew 1/2 A airplanes, little two channel planes with no throttle, (you just fly til it runs out of gas), then yes, we did mount the prop so it would windmill to the horizontal. These planes had no landing gear and were landed on their belly. It was easy to break a prop if you didn't mount you prop that way.

As far as I know, there is no specific position to mount the prop to counter any vibration of the spinning crank or to counter any micro surge in rpm at the time of combustion. It would take someone way smarter than me to prove the math on that one.