A few weeks back I was in company with some folk and one of them sung a song and another asked if they might have the words. The singer said that they had spent a lot of time and effort researching the song in archives and libraries and didn't really feel like just passing on everything to someone else which I felt was a valid point. However where the song is widely available it is different and I do sing songs I have heard others sing regularly but tend not to do them if they are present. That said there have been occasions when someone has asked me to sing a song they have heard me do elsewhere in company of the person who usually does it (unknown to them) I would then ask them if they objected or politely decline.