That is correct, for example my water is considered soft, so I must add some Concentrace to my filtered water. It is a good idea to check you municipality to understand what you are dealing with (the reports are public record and can be viewed online), also it doesn't hurt to have your water independantly tested, Lowes or Home Depot carry water tests that you can do on your own that are affordable!
The tap water quality may be fine but what of your homes pipes? This is why testing your homes water supply is important. Should there be any concerns you should consult with an environmental specialist to determine how to best deal with your water treatment plans. Sometimes you can get free advice from you local water treatment facility.