When I was a kid my parents took me to the 1st Nat'l production of Annie in Chicago. When they announced the understudy would play Annie, the audience made a LOUD groan!! Their reaction made me upset and I thought we were going to be short-changed with a second-rate performer. The opposite happened. I was BLOWN AWAY by the performance!! Instead of thinking, "Wow, how good is the "real" Annie?" I thought, "Why isn't she Annie ALL the time?"
I'm glad I had that experience at a young age because since then I've never worried about seeing the understudy.