It really would depend on the quality and profile of your tap water and the type of tank you are operating.
If you are running a simple FO system and doing high volume partials, RO may be effectively wasted, or may leave you open to unwanted algae blooms.
If you are running FOwLR and a skimmer and perhaps a macroalgae refugium, the lower level of water partials may well fully justify the extra cost and time to do RO or perhaps even RODI.
If you are running a reef, RO is likely to be fully worthwhile. RODI may even be justified, depending on the profile of your water source.
RO = Reverse osmosis
RODI = Reverse osmosis followed by deionization.
Where's the fish? - Neptune