I generally use the crib sheet on the first occasion, at least, that I sing a song to an audience, even if I think I know the song. In such a case, I'll have the crib handy in the front of the folder.
I don't usually know exactly what songs I'm going to sing, sometimes right up to the moment when I start singing, because I like to complement or contrast with the stuff that's been sung or played before me. It may be that the most apposite song requires me to use a crib sheet, in which case, shock! horror! I might spend a moment looking for it.