Per the 2008 SQL Licensing Overview:
The Business Intelligence components for SQL Server 2008 include Analysis Services, Reporting Services, and Integration Services. To use any of these components, the server on which Business Intelligence is installed must have a valid SQL Server 2008 license. If these components are in a separate server than the main database server, then they require an additional license of each additional server where they are installed.
For your case you should be able to install SSRS on your primary node assuming its licensed and be good to go;-) Personally I don't think its ever a bad idea to consult with MSFT first when it comes to licensing... better to reach out for understanding and ask for assistance than to keep silent, potentially be audited and then say "oops".
Here is a link to the 2008 pricing and licensing: