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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Until the transmission planetaries blow apart. For all practical purposes, most auto trans when put in neutral and driven from the output shaft will spin the planetaries to very high speeds. Would you drive 65 MPH in first gear?

sounds scary, but I've never heard of any problems as long as there's enough oil and the pump is running (i.e. engine on).

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