Since many Pinsiders are in Texas, I'll give my 10 cents worth.

Do you nudge and shake your games while playing them? Leave room for that, as the sides of the heads touching will cause rubbing damage.
Do you have many players over to enjoy your collection? Leave room for hand movement between the games. At the backbox leave around a five inch gap.

If you are the only one enjoying the games, and you do not nudge or shake the game then set up the games with at least a 1 inch gap. You will need room to slide the games out for service.