It is generally recommend NOT to tamper with a SharePoint database. Seems like the warning not to remove the label from your mattress. :) However, it is safe to run several SQL Server 2005 Maintenance Plan Tasks against SharePoint databases such as:
- Check database integrity
- Reduce a database
- Reorganize an index
- Clean up the history
- Update statistics
- Rebuild an index
Source: http://support.microsoft.com/default.aspx?scid=kb;EN-US;932744