Why on earth would you ever apply your lips or tongue to a girl's anus?
You can be a rock star in bed without rimming. You don't have to rim to receive love, admiration, or zombie-like devotion from a girl.
At the very least, if you're going to do it, make her do an enema beforehand. Clean out the plumbing and minimize the risk of bacteria. Even porn entertainers will tell you that you sometimes get "chocolate chips" from the poop chute.
I'm guessing some of you rimmers aren't requiring enemas from your girls (heat of the moment and all that). I assume you understand how waste travels into and through the colon en route to the rectum and anus. Just because you don't get a mouthful of feces doesn't mean you're home free. You're still getting fecal material into your mouth.
And that's assuming your girl's anal sphincter is working flawlessly, almost completely sealing off the chute. But that's highly unlikely.
In a word, disgusting.