I'm not sure at the percentages your using, but running coco with about 30% nutrient in it I find it helps to flush once a week. I started doing it with tap water, and then rinsed through the ph / nute but, after checking my run off after a ph / nute flush, I found it was miles too high, up near neutral. I'd only started flushing about half way through the flowering cycle and had been using drip emitters (which were allowing the media to dry out - now using drip rings, problem solved). I had to pound the pH back down with huge amounts of low pH water, a good amount of it was around pH 2. I'd think about it more in terms of media mass. Whenever you're growing in a pot, there's always a huge amount of material stored up in the media, a flush seems to freshen things up a lot. I'd flush. Make sure you use pH'ed water at least and check the run off. If they're outside and nearby, flushing should be easy. If the media dries out, the first lots of water through might not shift much because it needs to dissolve it out. It's best to flush once, then wait a few minutes and do it again. -like my reply, pls vote in my bulb poll (sig)-