Are you talking about the service account used to run SSRS as an application?
Or are you talking about the accounts used at the data source level to run the SQL within the reports?
The service account is granted the necessary rights at installation time when you pass in the account, if you want to change the account run through the SSRS setup wizard and change it within the service account section.
For the data sources, that's up to how the reports are written. May need select may need execute depends if using direct SELECTs or EXEC PROC commands.