Thayer hit the nail on the head. Shutting off fuel flow is only one part of the fuel efficiency equation: the engine will be braking the car, thus robbing you of your precious momentum. If the story ends at the bottom of the hill, then fuel cutoff is probably more efficientbthan neutral; if you have to go back up a hill, neutral may win if it saves you having to accelerate back up to speed.
However, if this is how you spend your brain energy, you're probably a good candidtate for an electric vehicle with regenerative braking.

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Prost!