I water with plain water when I think the PPM's of the bucket rez are too high.
Extra water is allowed to runoff and in this way the excess nutes / salts are removed from the hempy bucket.
I dont own a tester yet, but if you do you can check the PPM's of your runoff.
If its below 1100 then your within acceptable nute level for hydro.
Flushing to remove all the nutes in unnecessary. But to remove extra nutes that have built up is a good thing.