I agree with the view expressed above by some that it's reasonable to have a crib sheet, to be referred to if necessary, especially with a song that one has just learnt; but that to read the whole thing should be beyond the pale.

As for looking at a piece of paper while someone else is performing: I am sometimes guilty of that, when I'm consulting my list trying to decide what to sing or play if I'm called on next. I often don't plan in advance, as I should, to get a shortlist of possible songs and tunes. That need not preclude a last minute choice of something else, inspired by a previous performer.

Richard