With fuel injection engine should be using no fuel when engine braking... (with RPM well above idle).
It will use fuel when idling (even if you are idling while in gear with the clutch engaged).
That said, if you are engine braking you slow down faster than if you are coasting, slowing down costs momentum which you need to use fuel to get back...
I'm sure there is some perfect algorithm for when to coast and when to downshift for the best fuel economy... I don't know (or care) what it is