I flush because I use tap water, too much salt build up along with feeding every watering. I dunwana taste dat shit! People that do flush say start 2 weeks before you harvest. I think that is really only if you flush with distilled water. If you are using a flushing solution such as "Final Flush", you can feed up till 3 or so days before chop, then flush.
I flushed hardcore this time just because I felt there was so much salt build up. I started flush at week 6 of flower on an 8 week strain AND used "Final Flush". I'm now gradually giving them distilled water with "Bud Candy" at 4x the recommended dose for a sugar soak to see if it will help bring out the terps and trichs more. I also toned down my flowering time to 10hours on and 14 hours off instead of 12/12 for the last 2 weeks, have let my cold intake run when lights are off and will be inducing pure darkness the last 2 days as well as the final sugar soak, makes for a longer drying time but they will fatten up more and the trichs will come out more. Hoping the flush and sugar soak really do the terps well.