As far as the purist approach of adding margins to specific paragraphs goes, it's fine for general margins across the doc, but for softbreaks specifically I think <p> </p> is the best solution. This is primarily because of the possibility of format shifting. <p> </p> is MUCH less likely to be ignored during a format shift, whereas css margins get lost in translation all the time.
@cybmole, <br /> does not need to be paired with a closing tag because it's self closing. Stick with book view and just hit enter followed by spacebar for soft breaks and you'll get <p> </p>, which a fair number of users in this thread are voting for. <br /> is really meant to be used inside a populated paragraph, not an empty one, at least in the epub world where Adobe is currently the primary renderer on readers.