If you can't see it with the naked eye, it doesn't count.
But in all seriousness, I don't clean the case, just the screen, so there's nothing from my pant leg that's coming into contact with the case. The case doesn't seem to show fingerprints, and it's only a week old, so I've no reason to clean it. (Well, I've wiped down the back side a few times, but that doesn't show, so, eh, any scratches there don't count either!)