Could be that you're so used to using regular "tap" water that anything else just tastes out of whack to you. I do know that most bottled water I've tried tastes acidic (to me) and that could certainly lead to a sour taste. Your tap water could be great as is depending on where it comes from.
The main reason purified water is recommended is to lessen the chances of chlorine, lead, limestone (which will lead to scale buildup) and really a host of other things that can be present in most well or municipality supplied water.
Try this... buy some pool test strips, the type that indicate 5-6 things in your water such as chlorine, PH, alkalinity, hardness and test your tap water to see what it's showing.
Finding great water for coffee can be tough. The most common things used are the filter pitchers/faucet mounted type that really only mildly filter, leading to better taste, but not performance.
Some people think distilled or R.O. water is the best way to go but that actually makes water too perfect because it takes everything out. For coffee you do want a bit of minerals in the water to balance out the taste. You also want the PH to be in the 6.8-7.5 range as that is where water generally tastes better and is safer for all plumbing. Too low and it becomes acidic, too high and it becomes alkaline. You also want a touch of hardness, but not so much that scale becomes an issue.
Again, if your tap water tastes great for you and causes no issues with your equipment then by all means use it. Later!