Care to back this up a bit? Because I would think it's the other way round: the longer the ski, the less it matters (balance-wise) if you don't mount center.
Let's say you have two pairs of the same skis, 160 cm and 180 cm. You mount both at 2 cm behind true center. Now, on the 160 cm pair, the difference between the center mount and -2 mount is 1.25% of the ski's length. However, on the 180 cm pair the difference is just 1.1%. So if anything, mounting away from the true center matters LESS on longer skis.
Of course, if you wanted perfect weight distribution you would have to take additional factors into consideration: with non-symmetrical skis, the nose is usually wider and therefore heavier than the tail. Also the heel piece of the bindings is usually heavier than the toe piece. Now this leads to some pretty complex analysis before mounting, so I'm not gonna go there any further.
Also why would you even want the weight to be distributed evenly around your boot midsole? In a normal skiing stance, your weight will be on the ball of the foot, which is in front of the midsole. If anything, you should mount so that the ball of the foot is in the centre of the ski.