it's law, but some people just do it themselves. in that case it depends on the attendant and how controlling they are of their pumps. i've seen some people attempt to do it themselves and get shut down - always funny. "but i'm just trying pump my own gas!" (usually out-of-staters). me, i don't even try to squeegee my own windows because the attendants always freak out "no no no I'll do it" and i'm rarely in the mood get that "suit yourself lady" look, so i concede (i've been meaning to buy my very own squeegee to avoid this, but i keep forgetting). as for tipping, i'll only do it if something under my hood is smoking and a kind attendant offers to investigate.
as for doing it wrong, its not so much in the pumping, but i've seen attendants forget to put the cap back on the gas tank, or forget to close the little door.