this is a silly thread..
it's as silly as asking how you like to sleep at night.. left, right, stomach on your back..
point is.. they're all good. Electronics are not gravity sensitive!
This thread is not silly.
I can think of several valid reasons why things might be mounted one way versus another:
Mounting on the left side of the cabinet (looking at the front) might be better for running the wiring to the coin door as they are always (?) hinged on the left side.
Mounting on the right side of the cabinet (looking at the front) might be better because the power switch is usually on the right.
Mounting with the jamma connector oriented UP might make for a more reliable cabinet as the weight of the wire can't pull the connector off the board as it might if you mounted it with the connector oriented DOWN.
Obviously, it doesn't make any difference electrically as long as everything is connected. However, certain mounting choices may make for neater more efficient wiring, easier maintenance, or more reliable connections.
There are people here who have been working on cabinets for a long time and have seen what manufacturers have done wrong/right in the past, I'm just hoping to benefit from some of their experience.