What do you mean by "make the cut"? If I understand you, you're trying to create a half a head, and mirror it to complete the head, right? In this case, you want to have the vertices that form the center of the head profile running along the Z axis line. Or, looking at it from front the center of the chin, nose, forehead, etc will run along the very center Y line.
By the way, you do know that most faces are not completely symmetrical, right?
