I think the critical distinction which needs to be made here is between a Managed Folder Mailbox Policy and the Managed Folder Assistant.
The policies define the retention settings you wish to apply to mailboxes, the managed custom folders you want to create and so on. It is the managed folder assistant's task to implement the settings in these policies. If the assistant doesn't run, the settings are not applied.
So:
1) The policy is only applied when the assistant runs. Processing managed folder policies is a resource intensive process, so it is only performed when you schedule it do so (ideally, you would schedule it over night or when there is low load on the server).
2) It's not going to take any processing power if the assistant is switched off. The retention settings will never be applied.
3) If the assistant is running and you decide to stop it, use the Stop-ManagedFolderAssistant cmdlet at the Exchange Management Shell. You can specify a server to stop the assistant on using the -Identity parameter.
-Matt