I think an act should just play its set as a self-contained entity, and if called out for an encore, they should do what they want…something that’d be a treat for diehard fans. If the act doesn’t get cheered on loud enough for an encore (or at all, as I’ve seen) they shouldn’t come out. An encore is symbolic of the audience going mad for it, and you give them an extra little treat to say ‘thank you’.