It’s best not to use justified text. On very small screens this causes big spaces between words, especially if text ever wraps around images. Rather use any of Left (ragged right), Right (ragged left), or Centered alignment.
Note: Some intelligent ereading systems will override your settings either way, to provide a good-looking, easy-reading experience for the reader. But not all ereading systems are intelligent, so you should take the lousiest common denominator into account!