Well, I know I can shoot almost as well left-handed as right-handed. Fifty years of practice would improve that. Moreover, I know I can strike more quickly and accurately with my right hand. And as WarHammer 40K is about bashing things over shooting things, I'd take a righty fist. You are correct, however. And with a power fist quick isn't important or even possible. However, with a non-fist weapon, a knife/sword/ax/maul, I would definitely use it right-handed, as I know I am hopeless when it comes to using something like that left-handed.