flushing at harvest is a waste of time, a myth, and kind of stupid.
flushing when you have something in your pots that you don't want there is ok.
the actual practice is called "leaching" and the only time it's used in agriculture is when you have to remove something from your field. not from your plants. you can't remove anything from your plants once it's in there. if a field is mismanaged and over fertilized, it has to be leached to make it usable again. if it's contaminated during a flood or some other natural occurrence, they leach it, to remove the contaminants.
idiots at nutrient companies became aware of this practice and used it as a way to sell other idiots more of their useless shit products.
so, flushing at harvest, to remove anything from your plant, BULLSHIT. leaching to remove built up salts from your pot? perfectly acceptable, in fact, the only way to achieve your goal.