Well, I carry my G2 bezel up for the simple reason that that is the only way it fits into my BCC holster.
I have been carrying it that way for about two years now and have absolutely no scratches or mars on my lens. Of course, I have a Pyrex lens from flashlightlens, since I got tired of melting my Lexan ones. I also got a lockout tailcap to replace the original, since I bought my G2 before they changed to the LO tailcap. Seems that when I slouched in my computer chair just the right way, I would press down on the body and turn the lamp on. Since my holster has a very nice flap on it, I wouldn't see it one, and it apparently held in enough extra heat that my lens would bubble. Sorry, probably TMI for the topic.
As far as the discussion goes, it really doesn't make any difference to me up or down. I have to give the tailcap that half twist to unlock either way. I have developed a habit that is extremely rapid for my bezel up carry, and I can see how a bezel down carry would also be a very easy way to get into action.