To add to this, for the absolute most part the app will be accurate. However glitches or errors can happen. On my Wonder cruise back in October it incorrectly showed everyone's restaurant for night 3, so on night 2 the service team had to make sure everyone understood not to go where the app told them for the following evening and that we would be in the same restaurant two nights in a row. I can only assume those that didn't attend the MDR on night 2 would have had no idea and initially went to the wrong restaurant the following night.
Basically, rely on the app but also listen to your servers when they give you the rundown for the next day