There are performances, and there are sessions.

Each has its own etiquette. When I perform, I don't use
written material, except occasionally,only in less formal circumstances.

When I'm sessioning with other singers/players, I do use
written material, except for those songs I know well.

I'm not sure where this idea
that we should all try
to be performers comes from; past generations would gather
around the piano and sing from written materials.
It was understood that we were all 'amateurs'...