I would use softened water if I had it but if not, no big deal for a closed system. A softener does not add salt, it changes the type of salt in a water supply. Hard water is high in calcium and magnesium salts, mainly carbonates, sulphates and chlorides. If not treated, these salts "un-dissolve" (precipitate) and form hard deposits in the boiler, which reduces heat transfer and could cause hot spots. A softener replaces the calcium and magnesium with sodium. So now you have sodium carbonates, chlorides and sulphates (aka sodium salts) feeding the boiler. These sodium salts do not "un-dissolve" (precipitate) and do not form hard deposits. Other water treatment strategies, like pH control, further reduces the risk of these relatively low salt concentrations. Again, this is more of an issue with systems which have high make up, not so much with our OWB's.