Personally, I don't think you should need to say "excuse me." We seem to have something against flatulence which makes any situation like that awkward. I think saying "excuse me" just makes it even more awkward. So I won't say anything, and I would prefer other people don't either.
I just thought I would include that it's not that I think it's not rude, but I don't see why burping is less rude / more acceptable... they certainly seem the same to me. I don't think either is good to do in public, but at the same time, they're natural body functions, yanno? Why apologize for being human? (although still, I will definitely avoid it if it's at all possible!)