I'm very glad I don't have city water. I'd rather use RO and build up.
NOW, with that said, there is one very critical piece of the water that I can check, and I have found, it will vary.. pH. I check and adjust every brew day... When we haven't had much rain, my corrections take very little acid, when it has rained, which it has this past summer, it takes alot to over come the buffering potential. I'm certain the minerals are altered also, but it doesnt have as much effect as the pH, in my eyes. Sneezles61