Those that read the article on London water a few months ago, saw that the water in central London had a somewhat high sodium content through some its history. The porter breweries were reputed to make the most tasty beers for the style, even when compared to other porters from around the world. Having brewed that brown porter recipe that I created for the article and having brewed it with the 'London Porter' water profile, I can attest that the 110 ppm sodium content is not detrimental to the beer. If anything, I would say that you would want at least 50 ppm sodium in a porter to help round the flavor. For that reason, using baking soda is well suited to dark beer brewing.
Another consideration is that since you only add baking soda to the mashing water, the high sodium content that might be shown from the baking soda addition will be diluted when you add the sparging water. In the free version of Bru'n Water, you would have to perform that calculation by yourself. But in the supporter's version, the diluted ion concentrations that are produced in the kettle are shown automatically.