It's all about what's comfortable to you, but I was taught to keep my thumb on the middle of the back of the neck. It should be positioned between you middle and index finger. This also allows you to shift your hand if you need to do chords or runs on the thicker higher strings. THis would be the "classical" way to position your hand on the neck, but sometimes my hand will change and my thumb will go over, it just depends on what i'm playing. But you should defintaly start off this way and as you get better and learn to relax more when you play, you can do other things, but first and foremost is to do what is most comfortable to you. If it feels uncomfortable, or it hurts, you must be doing something wrong, that's the usual rule.