I don't think this statement is entirely true. Any grower with any experience will recomend flushing. By flushing, you remove alot of the chemicals from the root system thus improving taste and also the way the finished product burns. Im a soil grower and you pretty much just place your pots in the tub or I use a 5 gallon bucket and 2 wooden boards, set the plant on it and pour 1 gallon per pot size, of straight water thru it till drains all out. Repeat process when pot dries and that gets em flushed pretty well. Then just plain water from that point till harvest.
With Hydro, Im pretty sure if you have bubble buckets, you just drain the buckets and use straight water for the final 2 weeks. There are additives one can use to flush with. Just do some research.