Only on stick shooters. With a mouse, free-aiming is always more precise.
I generally take a dim view of people who use it in PC shooters, but if the game allows it, it's hardly cheating, I suppose. And there are some games where you can't fully turn it off- for instance, in Evolve Bucket's guide laser always snaps to enemies within a few pixels even if aim assist for guns isn't on. I think grenades may fudge a little too; I've made some impossible air shots with Abe.
