Shoot right above where the neckbone is, unless it's going through some crazy animation like a flip, in which case shoot where the neckbone will be or was midway through it's animation. Ex: Tamika is in flipping animation during a double jump, so shoot at the bottom of the hitbox regardless of where Tamika's head is in the animation.
I think it makes more sense being rewarded for shooting in an area above the neck instead of hitting someone in the forehead, becuase in models unlike juggernauts the rest of the body is exposed as well as the head so it doesn't make anysense why someone's forehead would be weaker than the rest of the body, unless it penetrated the bone or caused brain damage, which none of animations or particle effects in ut2k4 imply, plus, after the headshot, the skull of the character flies off and if it wa the sks a headshot, the skull would have broken, but the head flying off a character more implies that the connection between the head and character was severed.
The head in headshot is the only thing misleading about the headshot system in ut2k4, I think it's better than the alternatives available.