Quote Originally Posted by cowdery
Distilleries do not use distilled water when they dilute whiskey, either prior to barrel entry or prior to bottling. If the distillery uses spring water, that's what they use. If they use city water, it's typically treated--demineralized or what-have-you--but not distilled.
Demineralized water is presumably the result of reverse osmosis. The resulting water is all but the same as distilled water. That is to say, H2O and not much of anything else. Home units typically remove 95-99% of the total dissolved solids (minerals). I imagine that commercial work at the upper end of that.

Quote Originally Posted by cowdery
Personally, I use tap water run through a Brita pitcher filter.
Britas filter out much/most of the minerals via ion-exchange (substitutes hydrogen ions for metallic ions), depending on how new the filter is, though not nearly as much as RO filters do. It also has a carbon filter, which removes chlorine and other off-flavors. It's probably a pretty good choice for this.

Since moving to the country with a well 15 years ago, I have become very sensitive to the taste of chlorine in water and find it very unpleasant. I would certainly never use water with noticeable chlorine to dilute whiskey.

Our well water is fairly hard, so we buy reverse-osmosis (plus carbon filtered and UV disinfected) water from a dispenser at the local supermarket for $0.29/gallon. I had considered buying an under-the-counter RO unit, but I can't compete with this price, especially considering the maintenance I would have to do. I fill five gallon (19 liter) plastic jugs that fit on top of a ceramic two-gallon crock that sits on a wooden stand and has a spigot. I use this for drinking and making tea, and for diluting whiskey.