Yeah you don't need to ph as there are no elements that you are trying to make available. There is a buffer in there...in promix I think dolimite lime. The whole point of the flush is to rid of the salts. plain water will be fine. I have done this for many years will no ill side affects. your ph will still rise with out you ph'ing it.
What is your tap ph at? 7.5 range?
Ismokekush I run PBP nutes and they also work great. I have tried many and they all work, but the PBP is simple, organic, and not to pricey. Find something you like use.. and dial it in for your grow method. That is the key. Yes hydro nutes are they way to go as soiless is basically hydro, except you have a medium holding the roots in the pot.
My tap comes out at 7.1 average so I usually pH down till I hit 6 for the ProMix.