A friend of mine did this at my house once. I was pretty shocked, and I'm not a high stickler for manners at all--I never learned proper ones. I think my eyes must have widened or something, because she shrugged and said something like, "Yeah, I was born in a barn." I didn't say anything about it, and it certainly didn't affect my friendship with her or anything, but it was one of those moments. I found it pretty disgusting and don't think I'd ever seen anyone else do it in real life!