What is your goal? What do you want your pH to read? RO is soft and slightly acidic, and unbuffered, so the pH can quickly decrease or increase. What are you going to add to it to stabilize it? If you just add fish it will increase. Substrate, increase. Live plants, increase. Dead fibrous material, e.g. peat, Catappa, driftwood, etc, decrease.
I notice a troubling trend in modern aquarium keepers, where the measure of welfare seems to be steeped solely in terms of survival: if the fishes live, things are good, if the fishes die, things are bad. It is an inappropriate position to take. [Nathan Hill in PFK]