Engine breaking will often cause you to go into higher RPMs which will mean higher fuel consumption, which again, could end up costing more then what you save on break pads <-- especially with current gas prices
Actually, in modern fuel injected cars, no gas is used when you're in gear and not on the gas pedal since the momentum of the wheels can keep the engine spinning...and that's why a car will stall. Not enough momentum to keep the engine spinning, and not enough (or any) gas to keep the engine spinning. So fuel consumption isn't a problem with engine braking. It's the clutch and transmission wear that's the problem