There is a difference between 'prompts'or reminders and crib sheets as far as I'm concerned -
One is possible hints on verse orders, in the case of non-narrative songs say, the other is all to often a singer simply relying entirely on the written word.
The old habit used to be for a singer to stick a reminder sheet to the side of the guitar unobtrusively, not sure what unaccompanied singers did - problematical bits biroed onto palm of hand maybe!
I'm never sure of how a singer gets enjoyment from reading a text in front of the audience but I do know that I always found somebody reading in company simple bad manners.
Jim Carroll