I have a stupid question; does it make any difference vibration-wise if the prop is bolted on say horizontally when engine is at top dead center or vertically, or any position on the clock? Seems I heard once that a flyer claimed re-bolting the prop 90 degrees different made his engine run smoother. a spinning prop is essentially a disc isn't it?    

If it's a belt, or gear reduction the relationship is always changing. On a direct drive there could be a preferred location, or not.

If it is a wood prop, they say that you should store it with the prop horizontal so that the moisture content of the two blades remains the same for both blades. I have been told that if you store it vertical that the moisture content can migrate to the lower blade and cause a not balanced condition.This maybe an old wife's tale, I really do not know for sure.

A wood prop can get out of balance if stored with the prop vertical, especially if it's stored outside. Once it has run for awhile it will smooth out. Not pleasant though!

Thanks guys! Really cool info there!