For me, my water works best for amber styles of beer. pH almost always checks out spot on with an amberish/reddish type beer. When I want to make dark beers I use a little calcium carbonate in the water. When I want to make light colored beers I either adjust with acid or build from scratch.
My water comes from 3 different sources as well, and every now and then I will dough in and get a really strange pH reading on a beer that normally comes in perfect. But it is really very rare.