I sing and play from memory as far as my own repetoire goes, but I have no problem using a cheat sheet if I'm accompanying someone else and they're doing something I don't know well. I just did a festival gig with a fiddler friend who has written some outstanding tunes, but I only play them a couple of times a year. I'd rather have the changes in front of me so I get 'em right. If we had a "real band" together and played the tunes more regularly, I'd memorize them.