i am 100% organtic and i flush 1 time a month and 5 gallons for the last watering. it is best to starve your girl for a few weeks ( lower ppm) for the last 8-10 days i have adopted a no nutes standpoint. this is a clear taste prefrence i have had alot of plants harvested after 6 weeks still on full strengh nutes. the plant panics so to speak and will suck the green outta every non essentail leaf. this makes your smoke easier to cure. during the last week your plant will force more resin to the buds to attempt to save them. starving will signal the bud to stop growing and start preserving itself.
by regular flushing you will remove waste salts. this is the plants urine so to speak. with hydro you dont have this with your res tank being changed regulary but in soil these will make your flowers appear ripe to early.

for the one time flush i use ph'ed water till the bottom discharge holes run clean. then i do a heavy feeding the following day 1.33 ( one and one third) times normal ppm.