Let me preface my answer with this: our city has some of the cleanest water in the country to start with, then the water filtration system for the city is one of the most high-tech also. So our water is kind of amazing.
That said, our pedi said as long as we're on city water, there's no issue with using plain tap water. If we were on well water, she'd want it tested. When we're out and I'm using the powdered formula, I use tap water in an Aquafina bottle.
You could always just use ready-to-feed formula, then you wouldn't need to worry about the water at all.