Retainers don't cover top 2nd molars, should I be concerned?

Recently I got a new set of retainers made. However I noticed that the top retainer(essix retainer) doesn't cover 2nd molars. The nurse told me that usually there isn't much movement at the back teeth so it's not necessary for the retainer to include those, but I didn't hear that from orthodontist himself so I'm not sure how accurate that is. Should I be concerned that the top 2nd molars aren't cover in the essix retainer? Or do they generally not move enough that it's okay not to include them?