One should never look a gift horse in the mouth.
Speaking of, what is the history of that strange phrase? I'm sure it pre-dates the Monkees. Are its origins based in the wild west, when people would give one another horses as gifts? Was it considered impolite to look in their mouths? And if you got a gift horse, why would you look in its mouth? To see if it had teeth? Gingivitis? Braces? Fillings? Or maybe something in a horse's mouth indicates its overall health, so you look to make sure it's healthy? Because you sure as hell wouldn't want to accept a gift horse that's going to die on you. Then you'd have to bury it, and bury a gift horse is a pain in the ass. When my speculation ceases to amuse you, look here for the real answer.