I only sing in singarounds or folk club singers' evenings but much prefer to sing from memory - sounds corny but I think I can concentrate more on the song when I'm not looking at the words, but hopefully singing them on 'autopilot'.
However a few years ago I invested in a PDA gizmo which took the place of my folkofax and made my handbag much easier to carry. Even if I have been called the gadget woman!
These days I often look at it to remind me of a song, especially the 'run' of the verses if it's not telling a story.
I do cheat with words on paper when it's a song for a special occassion, e.g. birthday,Christmas that I'm unlikely to sing again.