This is fine for a manual transmission car as the driver is used to quickly engaging the clutch and matching the engine's RPM to avoid stalling, but for an automatic and an inexperienced driver, could be dangerous.I find on long trips that putting the car in neutral when going downhill makes a difference. I once got over 100 km more out of a tank of gas simply by using this strategy during a trip from Toronto to Sudbury and back. There is a long stretch up highway 400 that is very hilly so every time I got to the top of a hill, I would put the car in neutral and coast down the hill - sometimes for up to 2 km at a time. Made a huge difference.
Do a google search on "hypermiling" to get other strategies for getting a lot more mileage out of a tank of gas.
Essentially you are out of control of your car since if acceleration was needed in an emergency, there would be a significant delay.
Mythbusters looked at A/C versus windows, and as well slipstreaming behind a big rig. For the second, you could be a good ways back and still be taking advantage of the reduced drag.