All water in the brewery should be treated (IMO). For the mash, we worry about mash pH primarily, and use mineral additions and acids to achieve our target mash pH (at room temperature). Treating the sparge with acid is to reduce alkalinity to a point that will not allow the mash pH to rise above 5.8 during lautering (preventing tannic extraction). Mineral addtions, and to a lesser extent the choice of acid (acid malt, lactic acid, phoshoric acid, etc.) also have an influence in seasoning the beer. You can enhance desired characteristics through different mineral additions. Finally, your recipe (grain bill) will have a very strong influence on your mash pH, and must be considered when treating your mash liquor.
Other priorities should be to remove chlorine and chloramines, even for extract brewing.
