I'm in the middle road on this one. To a court, I bet two to the chest and one to the head will look more like execution than self defense. You'd better be able to prove that the guy was still a thread at the time you shot him in the head.
There are a few quick counters to this argument. The first is that it is a standard technique taught throughout academies, the military, training schools, etc. The second is that if you've trained in the technique, you can be expected to revert back to it during the fight. The court does not expect you to excercise perfect, flawless actions, just reasonable ones. The third, and most central point, is that if you can demonstrate a reasonable and subjective fear for your life that you led you to believe that deadly force was authorized and necessary, it's not supposed to matter how you apply that force. Granted, individual situations will vary a bit depending on the prosecutor and your facts.