When you either delete or close off a ticket (INC or SR), it should then ask you if you want to close off any associated tasks or linked tickets. Otherwise, when the ticket is closed - the related tasks cannot be deleted unless the ticket is undeleted and re-opened. But also, the related tasks are still left open when the ticket it was connected to is no longer active.
We have it so that when all tasks are completed that it will close off the ticket but if a customer calls to cancel a ticket - we should be able to close off the whole workflow in one hit.