Replying to Armada Halo 2

So I bought the armada halo 2 skis. I brought them to my local shop to get mounted. I am using these for 100% park and I am ok at it but I want to get better. Well anyways I told them to mount them dead center. But when I went to pick them up they were at -2.5 cm at factory reccomended. He said that that is what they are supposed to be at for the park, since the skis have a rockered tip. I have a hard time believing that since shouldn't all park skis be center mounted? How much will having it back -2.5 cm effect me on rails? And I know for a fact Phil casabon has his center mounted, so should I keep them at -2.5 or remount them dead center?