Yes. You can add more "e-mail disabled" statuses like
if ($order['details']['BT']->order_status=='P' OR $order['details']['BT']->order_status=='C' OR $order['details']['BT']->order_status=='X') .... etc
return true;

But it is not absolutely neccessary, because you can always uncheck "notify customer" option when changing status. Sometimes you may want to customer be notified, this option will disable it completly.