For stock type horses, like QH, paint, and apps there poll is supossed to be approximately level with their withers. To be competitive, it has to be there. Obviously, it comes up a little when working over fences.
True, not every dressage rider does. However, in the higher levels, they encourage a higher, more "tucked in" headset.
I'm a dressage rider, obviously, and I prefer to see almost a hunt seat headset on younger, lower-level horses until they have the muscle to support a higher headset..