My 2 cents...
I've seldom specifically flushed my finishing plants as I'm one who grows in small pots of biobizz light mix and use maybe 1/4 to 1/2 strength nutes during bloom...simply feeding as the plant needs...
That being said, I only grow for my medical use and healthy plants.
My finished/cured nuggs burn smooth and ash is white/gray as an indicator I'm doing something close to right.
If you have several plants, pick some to do one way, a few another, and see which tastes/burn better...