Most modern fuel-injected cars can idle for over an hour on a gallon of gas, so saving a sip of fuel just doesn't amount to much. When you use your engine to decline down a hill, your ECU recognizes that the engine is operating at an RPM higher than idle, so fuel feed is nearly cut off at the injectors - some automatics completely shut off fuel until RPM drops closer to required idle speed. In a manual transmission car, I'm not sure how the fuel delivery system is handled.
In my own cars, I rarely use brakes on a downhill - engaged gears take care of that issue and both of them have "Manual Trans Simulation Mode" which allows the Auto to act like a manual. Pretty cool feature that does in fact slow the car to a near stop in almost all situations. This is how I slow down when leaving the interstate or even going downhill to maintain a certain speed best I can.