If Fred regularly sings The Dolphin or The Oxen Ploughing or whatever, it's common courtesy not to sing those songs yourself while Fred's in the room - and it might seem a bit presumptuous to do them on weeks when he's not there, particularly if he sings them well.

You can't really cater for this kind of thing when you're a visitor, though - who's to say what might be J. Random Regular's song? In Lewes once I did a Kipling song which got a slightly subdued reception, with some comments about how if so-and-so had been there he might have got in first with that one, etc. Nothing you can do to stop that happening, apart from going further afield with your song selections - although then you risk getting blank looks because nobody knows the song you've chosen.

The opposite problem arises when somebody who has a definite set of songs stops attending a session, or dies - in that situation you might well feel there's a positive obligation to start doing "their" songs.