I believe you will find that most people recommend that you use RO/DI water in your tank. Water from your tap, whether it be city or well water, is going to have some things you may not want in your tank. These can be chemicals, heavy metals, or other "pollutants" that are not going to be beneficial to a closed eco-system like an aquarium. Some pollutants in water can be harmful to invertebrates, such as copper. Other pollutants, such as phosphates, silica, and nitrates, can increase algae blooms.
RO/DI systems will filter the water to about 99% purity; labratory quality water. Then, you use your salt mix to give you repeatable results without worrying about what kind of things you are putting into your expensive fish tank.
Having said that, some people claim to always use tap water and claim great results. It may depend on the location and what types of dissolved materials are in the water in your location. You have indicated that things are going well with your tank, but you don't specify what is in it. One could argue that if it isn't broken there is no reason to fix it. On the other hand, I did a quick search on your previous posts. They seem to relate to "why do my crabs die" and "what can I do to get my water chemistry back into parameters". Perhaps there is a connection. "Not all well water is well water" (Cute line stolen from Kinetico water softener commercial.)
Most people on this site would probably recommend the upgrade.