I would agree that before performing people should learn the words, but I think it's acceptable to have a 'cheat sheet' available to glance at in case of a senior moment or forgetfulness brought on by a case of the jitters. I have occasionally been surprised at people who pull out a piece of paper and haltingly stumble through a song they are 'just learning' rather than singing something they know well and saving the new piece until they've practiced.