IMO Pre-harvest flushing is not only unnecessary, but potentially harmful to your plants. It places stress on the root zone by altering the amount of dissolved solids when the plant needs it most. I notice that my buds start packing on weight around the last few weeks. Why starve them when they are still growing?
Do you think that when you see the plant cannibalizing its fan leaves (Source of assimilates) that it doesn't affect the flowers (sink of assimilates)? If the plant doesn't have these (immobile) elements available, then certain metabolic processes are unable to take place. ie - It could affect the growth of your buds.
I've run Jack's Classic or
Dyna-Gro (1/2 strength a week before harvest) all the way up until chop. I had healthy green leaves and beautiful buds. With a proper cure, everyone who sampled raved about how flavorful they were. No complaints of fertilizer taste or black and sparkly ash.
I believe that black ash and herb that won't stay lit is attributed to an improper cure rather than "too much N stored in your bud."
You flush your media, not your plants BTW.