I do expect someone who has come along expecting to perform to have prepared, but it varies. I know many jazz singers who are happy to let the audience select from their huge repertoire without preparation. Often the only person given music is the pianist with swathes of white paper for improv. At open mikes even singer-songwriters don't seem to be able to sing their own material without cribs, increasingly electronic.At jam sessions, however, to join in an unfamiliar song it is useful to be able to look it up and join in on the second verse.
RtS