humans only use about 10% of their brain (if i recall right). I am sure the same could be said with a deer (more or less). Now since the whole head is not the brain (skull, hair, nose, mouth ect), you would need to hit a spot that takes up less than 10% of the head, not very good odds. also in one of my classes in school, we learned about a man that had a rail road spike go completely into his head and took out a large part of his brain. The man survived but suffered severe problems mostly about self restraint.
Given the odds and what can happen, i wouldn't shoot, even on a world record buck, i would just wait for another day or deer to come.