Also if you are on a center mounted ski, leaning forward is not nearly as important as with the traditonal mounting I have found. Personally I cant carve my center mounted skis unless I am in a true neutral stance, If I lean forward I wash my tails out.
As far as hand position goes though, I think as long as you know how to ski with 'correct' form, it doesnt really matter. When you are hitting the trees or the bumps, if you are ridin all steezy, you are doing it wrong. Hands up with pole touches for all mountain, hands low and in the front for park, as pictured. Standin tall switch onto the drop of the park.