No, you don't. Believe me, you really don't. This would require modifying
the Normal style to be Justified, which would mean that every other style
based on Normal (which is most of them, one way or another) would also be
justified. You really don't want to monkey with Normal style.
You can get the same effect by making another style, say Body Text,
justified. Then open Normal.dot, select the default empty paragraph, and
apply this style. (See
http://word.mvps.org/FAQs/Customization/CustomizeNormalTemplate.htm.)
Better still, leave Normal.dot alone and create a specific document template
with Body Text (or Normal if you absolutely insist) justified, and use that
when you want to type everything justified. See
http://word.mvps.org/FAQs/Customization/CreateATemplatePart1.htm
