Brewing salts alone won't make ok beer into great beer. And great beer is made all the time without salts. But salts can be a contributing factor to great beer...or at least 'beer that fits the style guidelines'.
One can buy a lot of purified water for the cost of a RO system. One can also just use one test result and guess the tap water is about the same, always.
In your part of the country, water typically comes from surface sources (lakes, rivers) and so can vary more than well water. 2 or 3 tests, winter, spring and fall, will tell you all you need to know about your water. It changes, but almost certainly not a lot.
The water here near Atlanta is very low in mineral content. Two tests a couple years apart were within a few percent of each other. That is a good situation, because it's easy to add minerals but really hard to remove them.
I add salts to every brew. A friend never does. We both turn out pretty good beers, but some are better than others.
The bottom line is: don't obsess over it. Throwing in a little calcium probably won't hurt, and it might really help, but you can't know until you try it. The stuff is relatively cheap.
.
