Most fishes today, can adapt to a wide range of water condition's so long as those condition's remain stable.
Some soft water species still demand soft acidic water in order to thrive, and other's still fair better in hard alkaline water.
Unless I was planning on keeping soft water species entirely,,I would not consider R/O unit, and focus on keeping fish that enjoyed the water I have from the tap.
Is much easier to keep fishes that enjoy your water from tap, than to adjust water to suit the fish.
R/O unit's aren't cheap, and it can take approx eight to ten gal of tapwater to make one gallon of R/O So you would need to store water for water changes or emergencies.
Also must add some mineral's back to the R/O water, or mix with tapwater for fishes health, and to help buffer the pH so that it remain's stable.
While pH may not be as important as hardness (in my view), it need's to remain stable week in and week out. Wildly fluctuating pH,GH,KH is recipe for sick fishes.
Unless your pH is lower than 6.0 or higher than 8.0 I would not worry bout providing R/O water.
Lot's of fishes can thrive with pH between 6.0 and 8.0.