^ what he said. And I should add that dunking your roots in plain tap water or any other medium that is drastically different in PH, PPM, or nutrient profile stresses the root system of the plants. Plants react to these parameters by adjusting how much energy is expended to actively transport minerals across the root membrane. Nutrients don't just flow through the roots, they have to be drawn in by means of systems that are quite frankly, complicated and beyond the scope of this forum.
In other news.. floroclean and other such "rinsing" products are the biggest fucking waste of money ever. It has been shown by other growers that flushing with these products has little effect on how much minerals are drawn out of the soil. Water is just as effective.
So in summary (especially for the guy who has images of dildos floating on his hard drive... You don't ever have to flush between nutrient changes.. its actually not beneficial, flushing products are completely bunk, and flushing for long periods of time prior to harvest is dubious. In hydro, a week tops, and make sure it is at the very end of the cycle as the loss of minerals will eventually cause a collapse of the chemical process that you are actually hoping for... cannibalism of nitrogen sources, conversion of chlorophyll, because the immobile sources of minerals necessary to complete those chemical reactions will eventually be exhausted.