well I guess it would depend on the water supply...
When starting a DWC I usually sterilise the 5 gallons of water by boiling (and I know that will de-oxygenate the water). Once cooled, then I pour it into the reservoir and inoculate with hydrogaurd. Any further top-ups and flush-refills, I only boil/cool the water during the first two or three weeks.
I do this because where I live, the water is fairly hard, so it has the trace minerals that the plants need, but unfortunately the municipal water supply is prone to algae blooms, usually a pink slimy algae, but in the summer we get blue/green algae blooms - and that can be nasty to the health of the plants.
During the first few weeks, even with Hydrogaurd, an algae bloom will settle and flourish in rock-wool/rooting-cubes & deprive the roots of oxygen.
I have been thinking about the viable of installing an UV aquarium filter and would kill even the Hydrogaurd, but I know that keeping the reservoir sterile is a challenge.
So essentially I do a hybrid method, sterillise the water added during the first couple of weeks and then inoculate the hell out of it with Hydrogaurd