Huh. I'd like to see how you throw that reverse hand. Only time my palm comes up is for an uppercut, and to be honest unless it's a boxing match I've switched to other tools before that one comes out.
I've thrown jabs both ways; if I'm focusing on boxing only, it would depend on my intent. The vertical fist (knuckles inside, unturned) is, to me, a little snappier; I'm likely to use it to lead to something else. The turned, horizontal fist feels like it's got more OOMPH, so if I'm throwing a jab by itself, it's likely to turn.
Of course -- I think most of the time, I just throw 'em, and don't think about it!
1. The jab can be and should be a knock out punch when thrown properly.