Are you brewing AG? If not, it won't matter. If you are, dark malts can lower the pH of soft water below the optimal range of 5.2-5.4 in which case you would add a mineral that will raise the pH of the mash. I wouldn't do anything to the water unless you know the mineral content of it and have the ability to check the pH of your mash.
Edit: Actually, I shouldn't say it won't matter for non-AG, but you don't have to worry about the mash pH for a non-AG batch. Certain minerals can affect things like the flavors of the hops, but for a stout I wouldn't worry too much.