We were there on 6/20 and had previously decided not to do the ADR for the reserved seats. The park was pretty busy and there was a second show at 10:30 but I was surprised that we did not have to wait in a line. Maybe it was just timing and luck on our part but we walked right in and the remaining third of the seats were still open. It was about 45 minutes before showtime and it did fill up completely to standing room only. I’d say don’t worry about it and you’ll be fine.
Also, I learned that you can rent space (such as for a small group) to watch it with tables, drinks, snacks, etc. I wasn’t aware of that until this trip when I went to get some refreshments, I noticed a sectioned area at the back with some candle lit tables. I went over and asked the CM working the area about it. I would think about it if I had enough members going at the same time. I think it would be too expensive for just a family of four.