I think it depends on where you live. When I used to live in Florida, the tap water there was terrible and tasted very cholrine-y. Even for our family, we'd buy water at the store, and if I had had budgies there, I would have given them bottled, filtered water, too.
Here, I'm fortunate to live in a city where all our water is local and from the mountains, so it tastes almost better than bottled water! Mallorn has drank our tap water the whole time she's been with us and has done fine
We never give ours tap water, but ours also had giardia in it at one point, and our birds ended up with it. We got rid of it, but we have never gone back to the tap for their water. We get ours in gallon jugs and I buy about 12 at a time so I have it for a while. We do the same for our cat too, and haven't had a problem since.