I have to wholeheartedly disagree w/ the "if the tap water tastes good, brew with it" idea. At one point, I had that mentality, and it nearly cost me the hobby! I drink my tap water all the time (Chicago, Lake Michigan) and it tastes great to me. So, I used it when I got into brewing. With one exception (a farmhouse ale), all of my beers had a weird off flavor!
Hindsight being 20/20, it's easy to look back and point at my water... but since it tastes fine for drinking, I was looking at all the other common factors. I spend hundred of dollars and many wasted batches before I tried using bottled water and VOILA! That was my demon.
When you're already spending $25-50 for a 5-gallon batch, buying your water for ~$1/gallon = another $6 is really nothing IMO.
One last thought to leave you with: I've moved to doing all grain batches, but I still go for extract batches when the recipe calls for it, or I just want to enjoy the convenience. My way of thinking is this: DME or LME is dehydrated wort, made from water that already had a water profile including minerals, trace elements, etc... so when I'm reconstituting DME/LME, I use distilled water. When I'm doing all grain, I buy "spring" or "drinking" water.
good luck!