Depends on the gun.
Most it doesnt matter since most arent tapered. Some its easier to do left to right. Just depends on the gun and the part in question since like some rugers and 1911's its right to left.
"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." — L. Neil Smith