I'm of the opinion that if you can memorise a song and perform it without danger of forgetting the words then good on you, carry on! If you are in need of an aide memoire, and not to have one endangers your 'performance' (in singaround or concert), then why not use one?
I have just been watching a recently released DVD of a well known singer-songwriter performing in concert. He is over 79 years of age and writes rather lengthy and complex songs. I'm pretty sure he is using a stage prompt of some sort in order to ensure he has the words correct and his eyes are definitely not making contact with the audience when he is singing. They appear to be on this unobtrusive prompting device. I have no problem with this at all.