But it is when you're in an unknown-to-you-previously environment and it's your turn, or you are invited, to sing. Are you really going to ask before starting if you you could be treading on someone's toes? You could just say, "this is one of my favourite songs, because...." and just go for it. if it happens to be someone else's "owned" song, that person surely has more songs up the sleeve?? (Ok, I do know a few people who only ever sing the same 4 or 5 songs, but they'd still have 3 or 4 left!)