The question was whether it was OK to use RO water /cal-mag to do a flush when finishing. It is accepted that cal-mag water allows for nutrients to be more available for the plants uptake. (that's what this threads abt) It is believed (not proven) that it helps with nutrient transport and use of nutes in the plant.

If you are adding cal/mag to RO water that has 0 PPM of TDS, you will only add enough to bring TDS to 200-250 PPM. This in in the OK to drink range, much like well water with micro nutes in it. Or better analogy, look at a bottle of Dasani, RO water with a small measured amount of minerals replaced in it for taste.

There is not enough of cal/mag in water properly done to leave behind any trace of it after the flush. Hope this makes sence or cents.
In my original answer, "taste" there was a reference to finished, dry bud.
OM