I was taught a long time ago that when I'm in the water.... if the mask is not on my face, then it's around my neck (under my chin), or less preferred looped on my arm if I need to get it off my neck for some odd reason.
I had to bite my lip a few times when my family was recently going through tehir OW class and I was doing the refresher class along side... at that shop they have a routine that if they see a mask on a forehead they immediately go into freak mode splashing the diver and screaming "Diver diver, are you ok?" over and over....
Meanwhile all the folks that were in the "in-group" when resting on the surface would put their masks on the back of their head with the strap on the forehead.
To me I thought it looked a little silly...like trying too hard to be cool... and I fail to see how a mask on the smooth back of the head is ok, but changing it 180° so the mask is on the smooth forehead is somehow not ok.