" Perhaps this is why nobody has a rim that settles with a valve stem at the 6 o'clock position, among other factors ?"

Rims don't have valve stems and the heaviest part of rim is usually where the manufacturers sticker sits... and this little bit of weight will usually drop to the 6 o'clock position.

When I got home I put my bike in the stand and watched my front wheel turn and when it stopped the valve was at 12 o'clock... the magnet for my computer was at 6 o'clock.