I guess it depends. For IPA my water is pretty much ready to go as is so I am not building my water at all, only adding a little sulfate to the mash. But for my pale beers such as kolsch I adjust the minierals in the mash but blend RO water with my tap water at a 4:1 ratio, RO to tap. In my case I am using the minerals mostly to adjust pH (and for a little mineral character as well) but I would not use my straight tap water to sparge my kolsch because I want all the water to be soft. Won't matter as much with an IPA.