I agree about being tuned before taking the stage but there is often a need to ‘tweak’ a string or two between songs that have ‘dropped off’ and, of course, those of us who use several tunings during a set will need to be able to do that ‘on the fly’. Nothing wrong with that - the secret is to keep the audience interested while you’re tuning. Most folkies become pretty adept at between-songs/during re-tuning patter.