Can I just say, if you read from a crib sheet you will never learn the words of the song. I know a guy who has sung the same song week in week out for more than 20 years and still can't do it without the lyrics in front of him.

That said, I usually have a lyric sheet in front of me because, at my age, now and again my mind goes blank & I need a cue for the next verse or line. At that point I take a discrete look down at the table always trying to make it look like a natural head movement or with half-closed eyes.