I object (generally speaking) to using song sheets since it's less likely the singer has internalized the song, its meaning and its "setting." If the performance is aimed at beautiful presentation, guitar work & emotional contact with the audience then words don't matter much anyway - might as well sing "doo-wah." On the other hand, if it's a ballad then the story and meaning count far more than getting the words "right." It really doesn't matter, eg, what color Alice's dress is unless it must rhyme (if it must rhyme with 'you' then singer will remember the color without a sheet.)

That said, there are many exceptions. And yes, even Pete Seeger has been known to use a crib sheet at a major concert. At least once. Seems he just came on a song and he had to share it. At our local club people often bring sheets and we even provide a music stand to help. But this club, though an "acoustic folk club," is a very informal round-robin and tolerates a very wide range of occasional material. Usually those reading are sharing a one-presentation-only song. Often they are showing their own material for reactions and comment. Often they're practicing for a gig. Often they have wide memorized repertoires as well. I'm considered unusual in never using song sheets but I don't write my own stuff and mosttly sing ballads which are easier to memorize than songs.

We also sponsor two monthly concerts and monthly house concerts (typically a one-hour featured performmer followed by a round-robin jam.) These are different. Now a "performer" is standing in front of an "audience," is paid (something) and is expected to come prepared. I've never seen a crib sheet in these cases and it would be considered tacky unless an acceptable explanation were given.

So - it depends.