We usually dressed for our dinners prior to attending the show. [We've had late seating on both of our voyages aboard the Magic and love it!]
One reason we dressed for the shows as well as dinner is the fact that there is usually a lot going on between the early show and late dinner. If you want to be able to catch any of the events between the two, say the family cabaret shows, family line dancing, or the numerous other events occurring in this time slot, you'll need to be dressed for dinner.
Just a thought.