Congrats Acorn, you've lucked into yet another religious debate. :-/
Executive summary... Some reckon that expecting people to learn the song by heart before they sing it is excessive. Some reckon that as a "just in case", they're acceptable, but you should know the song well enough for it to be an emergency measure. And some reckon that if you can't remember it every time, you shouldn't be doing it. It also depends on whether it's a pub singaround or a gig you've paid £50 for.
Me, it's always the first line of the verse that I lose - if I can get that then I've got the whole verse. So my songlist has just the first few words of each verse as a prompt, which saves space (30 songs per page) and still gives me the emergency reference source. If I'm being good, I check my memory *before* I sing.