It all depends on how the connection to the source data is defined. For a user to be able to run the report - they just need access in report manager to the folder/report. They do not need SQL Server access or added to any SQL Server roles.
For example, if the report data source is defined to use the users windows credentials - then that user needs access to the source database system and database(s). If the report datasource is defined to a specific SQL account an password, the user does not need any access to the source data for the report.
Report Manager is configured to use a service account to access the reporting services databases - and security for the site is managed within report manager only. Again - users do not need access to SQL Server to be able to access reports.