I notch.
As I stated in the "Biggest Myths" thread, the hackle coming straight down does not help float the fly, therefore it does not float the fly higher. Remember the pin trick? Lay a flat pin on top of a bowl of water - it floats. Place it on the water tip first - watch it sink.
Hackle isn't hollow either. It won't float like deer hair.
The fly is supported by the hackle coming from the sides of the hook.
The advantage of notching isn't to get the fly riding lower in the water (since the unnotched fly is riding that low already). Rather the advantage of notching is to ensure that the fly "cocks" properly when it lands.