I wonder how many people would willingly admit to be bad at blowjobs.

As for BJ, it depends on how close you are to the person. That's basically the only rule I have. It's rude to come in someone's mouth you know without warning (because of messiness and disease and whahaveyou -no sex without talking extensively about it first, man!), and if you're close enough to allow your partner to come without warning, you might as well swallow.