I dislike it intensely when singers announce that they have been given a song as if it confers special rights -or that they learned a song from the singing of -when in fact they mean they listened and learnt it from a CD like the rest of us- but it implies some studious superiority which is intended to be demeaning in a lot of cases. Many folks are alarming name droppers! Personally I will avoid singing songs that I know are in other peoples repertoires who are present and even avoid songs of those who are absent if I consider them a definitive version . I always use song sheets because I have a       pathetic memory and sing better if I know I have access to the words -this is particularly important since I sing in a harmony duet and the right words are crucial. I have never yet remembered the words accurately to my own songs in performance -call me pathetic but thats the way it is -I do not think it detracts from my abilities as a singer although I have had people lecture me on what bad form it is- ironically by people who have no singing ability whatsoever.