If you don't already know, you can't treat SharePoint databases like any other database in regards to SQL Server. Here's a listing of articles that talk to what you can do. Before doing any maintenance, PLEASE read these articles. Index fragmentation is a big hotspot for alot of SQL DBAs.
You experience issues with portal alerts in SharePoint Portal Server 2003 or with search performance in SharePoint Server 2007 after you create SQL Server 2005 maintenance plans http://support.microsoft.com/kb/930887