I'm still much newer at this than most ppl here. . ..but, my basic opinion on flushing, as it pertains to "final flush":
It's "good etiquette" like washing your hands after you piss. . ..IMO both are a waste of water vs. their efficacy, but it's not wrong to do. Just, not necessary. Unless you pissed on your hands but let's assume you didn't
One more thing. .. i think it promotes better bag appeal, as most plants will exhibit wonderful colors or a nice light overall coloring, i genuinely don't think this translates to "smoother" smoke just solely visual appeal.
That's it. Oh and this is coco, cannot speak for soil or DWC, yet.