Most of my guys are in vehicle mode, so at first I didn't think I'd be able to reply. But then I remembered that I have photos of many of my guys. So let's see...

Kup: Seems he's a lefty.
Ultra Magnus: He's duel weilding, but his BFG seems to be in his right hand.
Wreck-Gar: Righty.
Rodimus: Duel weilding, hard to say.
Arcee: Duel weilding, hard to say.
Springer: Duel weilding, gun in right, sword in left.
Blurr: Both hands on one gun, but the main handle is in his left. Lefty?
Wheelie and Grimlock: Neither is holding a weapon in this pic.
So in that photo... seems pretty evenly spread out.
Seems to be the case in other photos I have, too. (And many of those are solo photos, so it isn't just a case of what was necessary for the posing in the above photo.) For example, going through photos of my Optimi, the final tally (not counting any duel-weilding ones, or any such as the first movie's leader class Optimus where toy design dictated which hand held the weapon) was 12-11 in favour of righties. Lefties took an early lead with the G1 Optimi, but had a surprise last minute surge when I got to Animated.
I, myself, am a righty. My mom is a lefty, though. While I am predomanantly right handed (for example, I could never write with my left) I do figure I get a bit of ambedextarity from that, for reasons such as if I was sitting next to her in a restaurant as a kid, if I ate right-handed we'd be bumping elbows the whole meal.