First off, welcome to Aquaria Central! I hope you enjoy surfing the boards.
In the Marine world of fish keeping, yes, RO/DI water is an absolute necessity but in the Freshwater world it is not necessary. In fact in almost every case, tap water is preferred since it has all of the minerals and such that are needed in the aquarium water for freshwater tropicals and freshwater cold water fish such as goldfish and koi. You will have to add a declor to the water during water changes and when you are topping off due to evaporation. A good one to use is "Prime". If your tap water is to hard for the fish you want to keep such as angels or discus then you might want to do a mix of tap water and RO water so it is diluted and the fish will be happier.However, in most cases the fish will adapt to the PH and hardness of the water as long as it stays consistent. It just depends on what fish you want to keep. If you do use straight RO/DI water you will have to add back chemicals to the water since they are not in that kind of water( RO/DI ) and you will not need to add a declor to the mix since there will be no traces of chlorine or chloarmines.
So what size tank are we talking about here? What fish are you interested in keeping? How do you plan to cycle the tank? Fishless cycle or fishie cycle?
Marinemom