Where should the ski binding be on the ski?

Where the binding is mounted is all about personal preference.

If you ski terrain park, a center mount could be more your style, where as if you ski all mountain, you may wish to mount farther back. Many skis come with a "recommended" line. You may wish to go from there. If you have further questions, any tech at your local ski shop should be able to answer all of your questions, and mount the bindings for you!