Sierra Nevada uses phosphoric acid to treat their brewing water to 5.5 pH. I know they add salts to the kettle for some beers.
Stone has a large industrial RO system, and the blend 1:1 RO to supply water. There was a pallet stacked high with CaSO4 and CaCL2 when I was on that tour. I would say they are doing some tratment.
Water varies with location. If you look at the hardness and alkalintiy maps in the link, you will see different breweries will do different treatments for different beers.
Where were you at? One of the local homebrewers in SD last year said the water works great for IPAs, and that is why so many are brewed there.
Edit - the water on the Detroit water system is great for light amber to brewn beers. People who have it only need to consider treating the water for Pilsners or dark Stouts and Porters.
Water is the last thing to play with as a homebrewer. It can make a difference, but get your other processes under control first. Don't go overboard, less is more on the treatments.