I'm lucky - good, neutral well water.
I worked one summer for a really good LFS, the owner has 30+ years experience with fish and exotic animals and supplies OSU vet school with all their livestock. We always put prime in the water and thoroughly stirred before adding it to the tank. Besides being very knowledgeable, he was cheap. If he didn't think it was necessary he wouldn't have been buying gallons of the stuff.
It depends a lot on where you are, but some municipal tapwater has such a strong chlorine odor I can hardly drink the stuff. Hate to think about breathing that stuff...