Just put it in 5th and put cruise on.... I seriously bet you will use less fuel keeping a steady throttle position down hill than goign down it in neutral. Hell at this point you might as well turn the engine off too. You know that its illegal to coast in a vehicle in neutral like your saying don't you?

Seriously just leave it in gear... the torque converter on the auto allows the RPM's to drop when less throttle is applied, using less fuel.