I know this is kind of a newbie question, but what exactly does it mean when a horse rubs his head on you? I know even without bad intentions, it can be dangerous because its easy to get knocked around. Is it a display of affection? Or does it mean the horse is testing you for dominance? Or does it not mean anything more than that he has an itch?
Itchy. But don't let him, because it will turn into a lack of respect/intruding on your personal space.
When you are done your ride, bridle off, halter on, horse tied, then you can rub his face with an appropriate brush. Just make the contact under your terms.