at or mt, it's not advisable. even if you're in neutral, it's still connected to the transmission, the engine's still on idle. if you do save gas, maybe 1% lang or even lower. plus, it'll just add wear and tear to you your tranny, as it was not designed for that. plus, it's very dangerous, as it'll be harder for a car to stop pag naka neutral, than when it is engaged in gear.
better to put it in a high gear nalang, 4th or 5th, then depress the gas pedal ng konti.