This is plain and simple.Divers in distress will almost always put there mask on their forehead or take it off completely.There is no wriiten rule just common sense.This is a sport with a element of danger.It just makes it easier to identify a person who is in distress.I for one would not do it here in the north atlantic .I wouldnt take my mask off unless I had to.Just play it safe leave in on, turn it backwards or put it around your neck.Hopefully the day never comes when you may be in distress and someone just says "he always puts his mask on his forehead".Practice good dive habits all the time.Thats my view.Dive safe.