Remember this, keep your chin up, and your spine in a straight line. Don't put your ear to your shoulder, that will get you to start putting your chin down, and turning your roll mexican. Don't turn it straight to the left because you will do a flat spin. Don't think about your head position all that much. What is much more important is driving the board straight up the wake.
Here's a pic of a pretty much text book back roll position. Notice spine in a straight line chin up, handle down and slightly forward, and both hands: