When I ran Fox Farm nutrients, it was indeed important to flush during the transition, and the feeding chart recommended doing so several times in the grow. Transitioning, the FF line moved from using a veg mix to bringing in Open Sesame, their bud developing supplement, and it was generally understood that this was also a time when they transition the type of nutrition that was being supplied, and that it was best to clear any unused veg specific nutes out of the soil before moving on in this stage. Your nutes may work differently, but I recommend the flush. During synthetic nutrient grows where salt builds up in the soil, I would flush between 3-5 times, depending on how ambitious you want to be.