I have used baking soda as my go to mash adjuster for the darker styles for some time now and no ill effects from Na.
I have liked baking soda additions for dark beers, as well. I also have negligible sodium in my water, though, and I only use as much as I need, which isn't much with baking soda since it is potent. I'm not sure if I'd use it for a highly hopped dark beer, but it works well for the malty dark beers that I make.