When I did extract brewing, I always topped off with unboiled tap water. My city water is excellent, though, with no choramines (is that spelled right?) or sulfur-y smell. I even used my faucet sprayer to add the cool water, so that the wort was extra foamy and full of oxygen.
Keep in mind when you boil your top-off water (which is probably best anyway IMHO, even though I didn't do that), that you remove oxygen from it. So make extra sure to aerate by splashing and shaking to give the yeast enough oxygen if you've boiled it. Bottled water is a great choice- and you can refrigerate it to cool it so if you add it to 75 degree wort, you'll be about right to pitch your yeast.