It's not neccessary, you wont die. I've made an entire 5 gallon batch with 5 gallons tap water. It may give a slight off taste to the beer though, depending on your water and the chemicals in it. Boiling off water only kills bacteria, I don't think it will get rid of the chemical traces that may be in it. If you really want, call your local water supplier and ask about the makeup of the water. If you're just topping it off with a gallon, then you'd hardly notice.
Otherwise, if you really hate tap water, I get two 2.5 gallon spring water jugs from the market for about $5 total. If anything I'll add a little tap water to top it off, but you can stock up on water for pretty cheap, especioally if you buy a re-usable water bucket and fill it up for likr $.89 a gallon at a Wal-Mart or something