Well ideally everyone would know every song they want to sing off by heart (and easier to sing "from the heart" if you do). But there are maybe different levels? I.e. for public performance on stage, no words in front of you (although I can think of several groups of more mature years and maybe fading memory who do have the folder there for the occasional glance down reminder). But for sessions, so long as you basically know how the song goes ( the words scan, etc) and can sing it without your head down in the book, I don't mind people using lyrics.