As Stefan says, all Styles are based on the Normal Style. So, if you change anything in that Style, it can affect many other Styles in the document. This might be a good thing, it might not, depending on what your intentions are.

When MS used to produce printed manuals to go with Office, the Word manual actually included a chart that showed the Style hierarchy for the supplied Styles.

So, provided you understand what you're doing, there's no harm in using the Normal Style. Generally, though, there's no need to and, because of the risks of unintended consequences, it's good practice not to.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]