The water you use has a decisive influence on the quality and taste of your beer. Mash boiling works best in the acidic range with a pH value of the water of 5.5 - 6.5. A value of 7 would be neutral.
However, the darker the malt used, the higher the pH of the water can be, as these malts are inherently acidic. The hardness and salt content of your brewing water are also important. If you want to know more about this, you can find out more on this website.