PH-ing water is done so that the roots can assimilate nutrients from the soil. When we are flushing, we are doing so in an effort to rinse away nutrient salts, and force the plant to utilize its own stored nutrients. That said, why in the world would anyone PH their flush water? PH is irrelevant during flush.
PH during flushing in the veg and flower stage is important. It keeps the medium in the proper ph range. Flushing before harvest (2 weeks prior)... I guess you could go either way on whether to flush or not. I ph everything. As to comments about smoking harsh smoke by not flushing... Yes... I've personally had harsh shit by not flushing my first grow. I have heard that it's not as important with all organic grows, but i cant say from experience. As for scientific proof. Go find it if you can... I'll just accept the advice of an overwhelming number of advanced growers and people who have become true connesoirs by devoting their life practice to it.