To add to what Graham said, although you could format the Normal style as
Justified, you definitely don't want to do that because all the style based
on Normal (which is almost all of them) would then be Justified as well. A
good choice for your default justified body text style is the built-in Body
Text style. If you make it Justified, then the other Body Text styles based
on it will also be justified, but headings and other styles will not be
affected (though you may want to modify specific styles such as Footnote
Text). You can save style modifications to Normal.dot by checking the "Add
to template" box in the Modify Style dialog.