"centre" will vary from ski to ski, which is why i think it's a terrible reference point, unless you just go at a recommended mounting point for that particular ski. even amongst manufacturers and retailers, there seems to be a lot of difference between descriptions of the marked centre. Core centre, or cord centre, or one of many other names for it is more useful for traditional, non twin-tip skis. a straight line is measured from the tip to the tail (often using a string or cord). this number is divided by two.
personally, i think true centre is a much more useful and universal point. do some research for approx where you want skis mounted for your type of skiing. if you decide to use true centre as your reference, make sure the shop tech knows that though when you tell him where you want your binders mounted.
spark notes: true centre is usually further forward and makes switch riding easier (usually).