Jim C, am I really "equating using a crib-sheet with making an occasional fluff?"
Yes and no.
I don't think EITHER having a printed page in front of you and perhaps occasionally glancing at it OR making an occasional lyric flub makes your performance seem amateurish.

But we agree that "The best performers can have memory lapses, no matter how much work they have put in beforehand, just as they can lose the tune or have their concentration distracted. " And when that happens, it won't necessarily be a minor flub of the lyrics. Key parts of the lyrics or story could be omitted or twisted.

And, except for very informal song circles where the main aims are to socialize and to introduce new songs, I agree that the crib sheet should not be a substitute for knowing your song. I.e., if you are going to PERFORM, as opposed to just swapping songs with friends, you really should know the song you're performing -- well enough that the printed page, if you use it, is just kind of like the aerial acrobat's safety net.