Hm...I was just thinking about a public performance situation. In an informal setting, I'll use cheat sheets without the slightest qualms. And if I'm playing guitar, the chord sheet is always out! There are so few songs that I'm confident of that I don't dare try to just play something. But, I don't do any actual formal performance with guitar.
I also sing in a choral group. We all use music, because the stuff we sing is very complicated, and we're usually using our weekly two hour meeting to get cutoffs and dynamics and the nuances of the music down. If we have the notes and words there to rely on, we can concentrate on other aspects of the music.