CSS stylesheets at most levels of the cascade contain suggestions. (User-important stylesheet declarations do have the final say, though.) Sometimes these suggestions will override those at a lower level. Therefore, if you don't care about something, it's best (as an author) not to override the user's defaults. This allows the user to see an interface to which he/she is accustomed and can use easily. Stylesheets that specify too much can easily make pages unreadable.
There are a few exceptions to this rule. The most important is what I discuss about related properties. The other is the styling of XML when the client does not (or may not) know the semantics (meaning). (However, it's a bad idea to send XML with unknown semantics over the web, because user and user-agent stylesheets will not be able to provide the user with his preferred interface.)
(Up to User Stylesheets Guide, CSS, David Baron)
LDB, firstname.lastname@example.org, 1999-08-11