I'm a lefty too -- use the rotary cutter with my left hand (but use scissors with my right -- go figure -- all those years in elementary school either grabbing a pair of "righty" scissors" or one of the very few "weird" ones with the green handles meant for lefties, lol).
Anyway, I just do what feels natural in regard to placing/cutting my fabric, etc. It's been easier if I don't try to do it "leftie" or "rightie," but just "comfy."
:-)