2004 Ford Towing Guide recommends locking out OD if the transmission is experiencing excessive shifting between OD and the next lower gear. Otherwise, it's fine to leave in OD. But it also states to check Owner's Manual. I've read my 2006 cover to cover, and as far as I can remember, it has the same wording as the general towing guide.
The 2006 Towing Guide says the same thing, but then add this statement:
"If excessive shifting doesn't occur, use overdrive to optimize fuel economy."
These recommendations have changed over time. It certainly would not hurt anything (other than gas mileage) to err on the conservative side and lock out OD as a rule. But modern transmissions are much better at protecting themselves against abuse, so most now recommend lockout only if it's hunting.