Horse turning his behind on me?

Our trainer told us to never let a horse turn its backside towards us, and taught us how to make them yield the hindquarters in ground work. My daughter and I have discovered it's in the interpretation, though! If a horse comes up to say "hi" or is just walking nearby, and turns its back end while it walks away, is that bad? My daughter says they should NEVER turn the hind end towards us, no matter what. My thought is that if it is a neutral action with no ear pinning or anything, it isn't intended to be dominant or disrespectful. Which of us is right? What action should we take if one of the horses does that?