Its worth the investment without a doubt. I wouldnt add the high tds
tap water to a system at all, not top offs or water changes. Doing one and not the other is just silly, it makes no sense. If you go to the trouble of adding ro
water for one thing and then add tap water for the other you are wasting your money on the ro
You can get a cheap ro
unit for about $90 on ebay that is portable and a great starter solution, I myself went that route until i was certain I was going to stay in the hobby, then invested in a $180 unit.
There really is no way to get around it, tap water in the long run is just not good. If you are doing corals at all then its needed, if its fish only, well, some people dont care about the total dissolved solids they are introducing to the tank as its only fish. lol But if the end goal is to have coral then really really consider not using tap water.