It all depends what terrain you're driving and whether or not it keeps shifting in and out of OD. If you're driving flat terrain and its not shifting frequently (which it shouldn't), there's no reason not to use OD.
FWIW, even when I'm not towing, going up certain moderate grades around 50mph, my 7.3 will shift in and out of OD if I'm using cruise control. That's when I take it out of cruise.
SMB-less as of 02/04/2012. Our savings account is richer, but our adventures are poorer.