I stopped filtered water for my self years ago cause they filter the minerals out too. People as well as snakes need natural minerals. You can actually die if you don't maintain them. Like was said you can leave a piture out to dissapate the chlorine....But then once you put it in the bowl it does the same thing. I'm sure your snake doesn't jump right in the minute you give them fresh water.........If they do your a Baaaad owner!
Um...you are certainly not a bad keeper if your snake happens to like fresh water. I find that on occasion when I change water every morning, my snakes will come get the fresh water right after I put it in the bowl. Some do it, some don't. Maybe they prefer the fresher, cooler water as to water that has been sitting in the tub all night, getting warm. Who knows, but it certainly does not make someone a bad keeper. My corn snakes are notorious for this!
As for what I use, I would love to use filtered water, but I just use tap water for convenience and it is cheaper. We have a water filter we could use, but we would be replacing it far too often with as often as I change out water for all my animal kids.