If you absolutely have to get hit in the face during a fight, you can do a lot worse than the forehead. Granted, it's right where your brain is, but thanks to the wonders of evolution, that's okay. Your skull is designed to be able to take really tough impact on the forehead. Yeah, it'll hurt like a mother-fucker, but unlike the jaw, cheeks, nose, or back of the head, it's highly unlikely that you'll actually sustain any serious damage. Plus, there's a good chance your opponent will hurt his hand. That said, the skin on your forehead is easier to cut than most of your face, so there's always the danger of blood getting into your eyes. But your forehead is more likely to be cut by something grazing across it like a sidewards elbow or an arcing kick. A straight punch is really bad for opening up a cut unless it grazes the side of your head or something.