i dunno if he does it anymore, but charley ager used to mount his hellbents way the fuck forward (2-3cm i think), hence his insane switch skills in pow
from personal experience though, you're gonna want equal tip and tail, with center being at the mid-sole of your boot... the problem i've had is that i'm still unclear if the proper terminology is "true center," "core center," or "measured center," and that was exactly the problem i had w/ my shop tech, who ended up mounting my most recent skis nearly 3cm forward... i ride anthems (a symmetrical ski), so at first i was like "fuck it, im gonna re-mount them at the end of the day"... 3rd run in, just starting to feel comfortable, roll-in switch to a super easy box feature, catch my tail, lam my brand-new skis, bust the back of my head wide open on a fucking flatbox, and bleed like a murder-scene all the way into the clinic...
moral of the story: don't mount forward unless all you're planning on doing is hitting and landing jumps switch, because it WILL fuck up your balance/technique on jibs