I'm in the west and don't use mine for rails, they're at 3cm back from center and thats pretty much perfect for me, for you I'd say anywhere from true center to 2cm back would be your best bet, for 90% park I would go center or 1 back.
if they arent symetrical, and you care about all mountain performance for that 10%, go 1cm back. but you could still go centered, which is what my personal preference is (although some people wont recommend it).
if it helps, i have k2 recoils (not symmetrical) that were centered, and i got smaller boots and slid the heel foreward, and now they are about .5cm in front of center. i can still ski all mountain, and occasionally a bit of powder if i fuck up my ski choice for the day, really well. its just a little harder to lay a perfect carve on icy runs, but its doable if you have good form and are comfortable with how the ski skis
This years blend has a -4 core centre and I would say with 90% park you don't want to be 2-4cm back, despite line trimming down the weight of the nose and tail the swing weight will feel a lot better chord centre to -2cm. Personally my choice would be to go -1 you will get almost all of the benefit of the symmetric flex that the blend has, the lowered swing weight of chord centre, the amazing ride forwards (I have a pair of 10/11s and love them) and its wont be a big deal for riding switch.
I ride the 2013s as a park ski, mounted at true centre, they still ski fine forward as soon as you get them up to speed, they are just a little more reluctanct to hold edge on a turn because they just want to pivot, at speed this pretty much goes away! I skied pow on them a bunch last week and was throwing 3s off small cliffs and skiing some steeper more technical lines on them no problem.
I would definitely just go true centre for 90% park.