In our experience, with our local water supply, we never modify the water. Nothing is added. We do filter the tap water with an RO unit, or a countertop filter.
Maybe we are lucky and our water just happens to be perfect for brewing.
For a pale Czech style beer, we use 80% RO water, and 20% filtered tap water.
For a German lager (think Frankfurt - Bitburger) we use 100% filtered tap. Nothing added.