Actually dechlorinated Tap water is much better for your fish. If you read the FAQ sticky on the betta care part of the forum it specifically states that distilled water is the most dangerous for fish.
So you did the right thing. =] As long as you bought water conditioner that removes chlorine, chloramines, and heavy metals and you're using it. =]
If you haven't already read the FAQ do so. =] I read it before I got my fish, followed all the instructions and other than a case of finrot which is healing, he's doing great! =]