RO/DI have almost no advantages in freshwater, mostly drawbacks. When you run tap water through the RO/DI, it strips all micronutrients from the water. If you needed saltwater, then the salt mix would replenish all the micronutrients. In freshwater, there is nothing replacing those micronutrients unless you buy the mixing formula, which is a PITA when its not needed. The fishes and plants need these nutrients. Your tap must really have toxic levels of nitrate before considering RO/DI. If anything, I would run the waste water from the RO/DI unit into freshwater tanks.
When referring to Ammonia/chloramine remover, I think you must be talking about Seachem prime. Use it every time you do a water change. Most freshwater hobbyists use this as its beneficial to the fishes.