I teach for PADI, and have never seen any written standard on whether or not to place a mask on your forehead, frontwards or backwards, or whether or not to wear it around your neck. On the forehead frontwards, it could be interpreted by some as a diver in distress, but that would probably not be the first or only thing to indicate distress. Instructors have their own teaching styles and opinions, and they are not always consistent with each other, especially if there is no standard specified.
I have seen divers lose their masks by putting them on their foreheads on more than one occasion. I have also had mine knocked off by a wave in my early days of diving when it was on my forehead. Here are some things to consider when making your own personal choice about wearing it on your forehead:
1. It may get knocked off and lost.
2. It's the most convenient place and easiest place to wear it on the surface, and it doesn't get tangled up in hoses or on your BCD somewhere.
3. It doesn't give you the feeling of being choked by having it around your neck.
4. You don't have to stretch your mask strap so far out of normal shape to put it back on as compared to wearing it around your neck. It's also quicker bringing it from the top down.
5. It may be interpreted as a diver in distress if on frontwards.
6. HERE'S ONE MOST DIVERS DON'T CONSIDER...When you put your mask on your forehead, your body temperature and the sun will heat up the glass very quickly. Remember, your body loses most of it's heat from your head. Even a warm hood combined with the sun will heat up your mask. When you put it back on and the mask touches the cool water, it will have a natural tendancy to fog up. This heating should be slightly less if you wear your mask backwards.
7. Wearing it backwards feels more comfortable. You have the flat strap against your forehead, instead of the uneven contour of the mask frame. The glass is in the relative coolness of your wet hair, or back of your hood, not up top facing the sun.
In class I teach my students to wear the mask on their face until they are out of the water. If they take it off for some reason, I advise them to hold it, or wear it around their neck so it doesn't get lost and have a greater chance of fogging.
When I was in the Instructor Candidate Course (IDC), our much older and wiser Course Director told us not to wear our masks on our foreheads, and not to try to look cool by turning them around backwards. He was also very strict about tanks standing upright when not being used. They had to be layed down on the ground, which was a very good idea!
In real life diving situations when I'm not teaching, I have tried wearing it on my forehead frontwards and backwards, wearing it around my neck, holding it, letting it hang from my forearm by the strap, and clipping it off to a D-ring. I think the decision is a personal one that compromises safety with practicality and comfort. I had rather clip it off with a D-ring as a first choice, but wear it backwards as a second choice. Still haven't been consistent with it though in all these years, except for teaching! Think I'll try the D-ring again for a while!