hey sam, im just finishing up my first bag seed grow, bout to harvest monday! I started flushing in my 6-7th week on a strain that i expected to take 8 weeks. When I started, about 25% of the pistols were changed and I hadnt started looking at the thrichs by then.
Flushing for my plant (in about a 2.5 gal pot) involved a watering of about 2.5 liters each morning. My plant needed this much water on a daily basis partly because the humidity is dropping because winter in around the corner and because my plant is in a smaller storage space with an electric heater blowing at the base. But. . I definitely noticed my buds plump up a bit after a few days of flushing.
I'm no experienced grower (yet) but I definitely see a need for flushing if your using any kind of artificial nutrients. If you've ever got some bud that sparks like a sparkler when you light it, that because that bud still has a bunch of nutes and salts in it. To me that's just gross and usually comes with a bad taste as well, but I've heard some dumb asses get excited about it.