Using idioms is a double-edged sword. One side lets you cut to the point. You say a phrase, and it's so loaded of meaning, it's a shortcut to what you mean. One side can cut you, if you just use it and you use it wrong.
Let's consider Beatriz Sarlo, a respected argentine intelectual. She writes op-ed columns in a newspaper. She wrote one today. It ends:
Total, Boudou, sin bromita alguna, debe adecuarse a lo que le toque, obedeciendo el viejo refrán de que a un caballo regalado no hay que examinarlo para ver si viene completo.
Which I translate as:
In any case, Boudou, jokes aside, has to accept what he gets, followingthat old advice about not examining gift horses to see if they are whole.
One peculiar thing here is that we have almost the exact same saying in spanish and english. In english you don't look gift horses in the mouth. In spanish you don't check the teeth of gift horses. The problem here (if I may be so pedantic (yes you may (thanks other me! (my pleasure)))) is that Ms. Sarlo has no freaking clue of what that means.
Suppose you are buying a horse. You would check the mouth because you want to see if the horse is young or old (citation needed? Here is google's first result). That's important when you buy a horse. It's, on the other hand, really useless and rude when you are getting a horse as a gift:
Nice guy: Hey, here's a horse!
Rude moron: (looks in the mouth) No thanks!
That's why you don look gift horses in the mouth. And that's also why you don't check amazon to see how much the book you got costs.
On the ther hand, if you were a Tamarian, you could look at the horse's mouth and say "Temba, at rest".