Completely personal preference. Depends what skis they are and what you are using them for. Personally, I ski center mounted or at most 3cm back from true center if it's a directional ski.
1) Are your skis directional or non-directional?
2) Are you skiing park, pow, all mountain, etc.?
3) How often do you ski switch?
4) Cliffs? (I like center mounted for cliffs because I have a little more tail to help keep me from getting to far back/backslapping on landings.)
A lot goes into it. It's all personal preference. In my mind, it makes sense to have all your weight on the center of a ski...it's logical. On directional skis though, you don't want to be too far forward of the narrowest point of the ski.
√ Opinion given.