Leave "store" field blank but required when creating a new order.
When creating a new order the "store" field defaults to whichever store comes first alphabetically. We use this function to separate shipments by different departments within our business. By doing this we can track shipments by department very efficiently. If the "store" field were to be left blank by default (but still required) it would ensure the shipper is paying attention and selecting the proper store before creating the order.