See also: http://msdn.microsoft.com/en-us/library/ms189964.aspx
SQL Server 2008 Books Online (March 2009)
Service Account (Reporting Services Configuration)
A least privileged account obviously.
The account you specify for the Report Server service requires permission to access the registry, report server program files, and the report server database. All permissions are configured for the account automatically when you use the Reporting Services Configuration tool to set the account. If you use the service account to connect to the report server database, the tool creates a database login for the account and configures database permissions by assigning the account to the RSExecRole on the SQL Server instance that hosts the report server database. The report server database is the only data store that a report server writes to. The service account does not require permissions to any other data stores.
Use a built-in account
Select Network Service, Local System, or Local Service from the list. Only Network Service is recommended; however, you can configure the account to use any account that is available.
Network Service is a built-in least-privilege account that has network logon permissions. This account is recommended if you do not have a domain user account available or if you want to avoid any service disruptions that might occur as a result of password expiration policies.