I usually flush with tap water for the last 2-3 weeks of flowering. That's just me, though.
Plenty of very skilled growers (i.e. Uncle Ben) are proponents of progressive harvesting ...i.e. harvesting buds from the top down not all at once. Let the lower buds grow out after you've harvested the topmost buds. Obviously, you keep feeding until the very last bud is harvested, so no flushing. I'm going to try this on a large snow white I'm about to flower.