I have to admit, I'm starting to rely on a crib sheet for songs that once upon a time were fixed in my memory... but then I don't sing as often as I used to, and my memory is like a singularly volatile sieve. If it's ever my fate to succumb to Alzheimer's, it will be about 18 months before anyone notices.

However, some useful tips I learned at a dance callers' workshop many years ago:
   Rule 1. Make sure you know the dance thoroughly so you don't need a crib.
   Rule 0. Have the crib handy anyway.
Maybe the same consideration applies.