Tap works for me and I'm picky, but I recognize that not everyone in the world lives somewhere where the tap-water is usually better than the bottled water bought in stores.
Mineral content in tap-water can vary, even between places where tap-water is very well potable. Minerals can affect taste, according to [1] (I didn't check myself).
Oh right, I've not actually tried altering the water - I really should give it a go sometime. I just use tap water - the water in my area is ok. That's another one of those things that may make a bit of a difference, but nowhere near the difference that getting good coffee and measuring properly will make. Of course, if you're already doing both of those then there's no reason to give it a go. Thanks for the idea.
I buy bottled. The taste is better and consistent. If you go this route, make sure to try different brands (they use different water sources, so the taste will vary).