[Doug1]

Hopefully someone educated in Chemistry will chime in but basically as I understand it since RO/DI is fairly pure with no minerals or anything in it, it doesn't react to pH tests like tap or well or bottled water. Since you are replacing water that evaporates on a fairly steady basis there should be no big swings. keep in mind that you are going to lose some salt to salt creep, and if you run your skimmer wet there will be a slow decrease in salinity over time, another reason to do regular water changes and monitoring.
Also remember to disable the ATO when doing water changes so it's not running and dumping pure water or Kalk water to make up for what it sees as lost water volume.