Nitrogen is needed through all phases of plant growth, even through flowering and "ripening" but not as much as when in veg, the reason ChaChing has a low concentration of Nitrogen and not a high one. Flushing, weather or not you do it, its not "growing" its part of the curing process, so no you would not use Nitrogen during a flush, but you usually dont use any plant foods (0-0-0), only l-aminos, carbs, and/or a flush solvent.