The flushing valve is for cleaning the filters by opening allowing the water to flow through with minimum restriction. When the valve is switched to the off position, it isn't actually off. It still allows water to flow out, but at a reduced rate, thereby pressurising the system. If you have a pressure gauge fitted, you will notice that if you have the RO unit running, then open the flushing valve, the pressure will drop.
If no water is flowing into the system then you should get no waste water, obviously. If water is flowing into the system then yes, you should get a steady stream of waste water, regardless of whether or not the flush valve is open or closed.
HTH,
AK