In most cases, topping off with pre-boiled tap water is just fine. Bring a few gallons to boil the day before your brew day and put them in a sanitized container. Of course, knowing the actual chemical composition of your tap water would help and certain styles of beer do better with adjustments to you tap water. There are water chemistry articles here on homebrewtalk to learn more.
Beer so good, it's frightening.
2014: Seven Heavens Series One: Vilon, Mystic Melomel, Skeeter Pee.