I think you'll find that coasting (to quote an online dictionary) is "To move without further use of propelling power.". This means in gear or out of gear is irrelevant. Being in neutral or dipping the clutch, its the same thing and it will cause you to fail your driving test as it is dangerous. Each to their own, but I think its damn dangerous, and you must be very confident of all other road users if you think you can judge when its safe to do, you never know where a danger is going to come from. Try doing some safe slalom in the car when you are in neutral, it behaves differently to in gear, and can be quite unstable.
Just letting off the throttle will cut most of the fuel to the engine and is meant to use less fuel than at idle as the momentum of the car moving can pull the engine around a little (with a little help from the engine) but certainly uses less fuel than coasting and letting the car idle while you do it. Why not try both for a month and report back on what you find?
Anyway, enough with the rant