Partiuamo dalla definizione di Farm Sharepoint:

"A SharePoint farm consists of multiple servers that all use the same version of the SharePoint product or technology. "

Da cui:

• If the server farm uses Windows SharePoint Services 3.0, you must install Windows SharePoint Services 3.0 on the report server computer.
• If the server farm uses Microsoft Office SharePoint Portal Server farm, you must install an Office SharePoint Portal Server.

Tratto da:

How to: Install a SharePoint Web Front-end on a Report Server Computer
http://msdn.microsoft.com/en-us/library/aa905869.aspx

e ripreso anche qui:

WSS and MOSS in same farm for Reporting Services?
http://blogs.msdn.com/gayanpeiris/archive/2007/11/21/wss-and-moss-in-same-farm-for-reporting-services.aspx