Jillian,
> Is there any reason to have databases -not- owned by the users: Postgres or
> root?
Yes. If you have a multi-user database, you will want to restrict what the
users can change. In that case, the user has to be different from the
database owner, as the DB owner can change permissions.
Also, if your database is going to back up a web site, it's a good idea for
the web server to be a user with restricted permissions in case the site gets
hacked.
-Josh Berkus