I've never used a chemical to flush the chemicals out of the soil, but I do a final flush with two to three times the pot volume, with properly ph'd water. My wellwater ph is 8.0, so I avoid using it for anything unless it's adjusted. It doesn't take long to form nanners at this stage, and risking them because I was too lazy or too cheap to adjust my wellwater is potentially counter-productive in my book. The risk is not worth the 5 cents it costs for me to use my phDown.

Cannabis is an annual, and I believe that if you are doing it right the roots never stop growing.