It's been my personal experience that hover pitch is still mid stick, wether i'm at 10 ft or 100 ft.
If I understand the question you're asking, why does it take more power to make the helicopter ascend to a different altitude, it has to do with thrust vs weight ratio. When you're hovering, the helicopter is providing only enough vertical thrust to keep the weight of the model in the air (so you're basically at a 1:1 thrust vs weight ratio). If you want to go higher you need to increase overall vertical thrust so that the thrust being applied is more than the weight of the helicopter it's self, allowing it to climb. But with our models, once you're actually at the altitude you wish to be at, you should be able to hover in the same spot with your stick in roughly the same area. (mid-stick, usually).

I've never noticed my hover point on the sticks increasing when i'm higher in the air. I doubt air density changes much at the altitudes our models fly it.