Using automatically assign is also good but it applies to the whole transaction.  The part I usually need to modify is the item itself.  I have not found a way to set a rule to make a line item on a sales receipt be a particular product or service.  If that is possible that would be awesome but from my understanding it's not.  I also have certain cases where multiple items are in a single transaction and may go to different areas so that doesn't work there either.