Let alone the memorizing issue, I find that even something I know really well will just disappear if I get flustered - so a sheet of paper available to get back on track is great. It can also be really helpful the first few times you sing in public at a song circle, just because it gives you somewhere to look & blank out all those scarey people looking at you.... I know, I know, you shouldn't blank your audience out - but just for the first couple of times it can be really useful!! Presumably the more you sing in public the less you need to have a panic-sheet or an artificial focus. So, I suppose my theory is that when starting out it is OK, but perhaps after a while it is lazy & isolating (maybe). But if its either sing+sheet or no sing at all then the answer is to be discrete with the sheet & carry on singing.

Kris