Just note that any impurities in the tap water will be absorbed by the rock, only to be leached back into the water over the year(s). Compare the TDS coming out your tap to what comes out your RO/DI. What are the results? Remember that the water will evaporate; the impurities stay behind unless removed. Guess where the impurities are being stored over the years.
In my opinion, the cost of using RO is far less that your time and pain of resolving the potential consequences of using tap.
I guess the question we should have asked is, how big is the tank and what do you plan to keep? For a fish only, not so bad to use tap and water conditioner for cycle. If you plan to keep reef, I’d only use RO/DI.