I would never center mount a ski that wasn't a true twin. It's simply not how the ski was meant to be mounted and in turn, can really mess up the way the ski rides.. ski's are shaped with mounting points in mind, and if you go and mount them outside of that mounting area.. what was all that shaping for?
just my 2 cents.. obviously, if your broke and stuck with an older pair of skis', you gotta do what you gotta do..
but if you are going to be buying new ski's and are asking this question, you should probably get true twin tipped ski's such as the Line Anthem.