Thanks to you both. I guess I'll rely on Perkin's suggestion, though I don't remember seeing that option last time I added a mobi file to the library; it just added it.
Another option for tweaking which occurs to me is to look for the class which contains text-align: justify; in its definition and add the text-intent: 1em; (and font-family: sans-serif; ) there. It's worked so far.
By the way, a little off topic, I've noticed that some of the tag definitions in calibre do not have a trailing semi-colon. Not a good practice in my opinion.