Every morning, I put down a bowl of fresh tap water for our cat, but she always goes out and drinks from the garden pond or from tainwater puddles on the terrace. She is 10 years old and in fine form.
my cat drinks rainwater too! and she has a frsh bowl of tap water (evian when i can't be bothered to do it myself!) but preferss to drink out of a skanky old plant pot...
I know a dog who is always given filtered water...and now he will not drink tap water if he has a choice. So, that is the only down side I have heard about regarding a pet drinking filtered water.