We do, but we tend to get our takeout from a handful of favorite restaurants, where the people are especially nice to us and give us special deals and extra stuff.
Technically, I don't think it is absolutely necessary since there is no "service" as such involved, but it is a nice gesture, and can be helpful in making you loved at that restaurant, and thus get special deals and extra stuff...
And if there is a "tip jar" by the register, by all means, you should drop a few dollars in!
And I am referring to pick-up take out. Anything that is delivered, I think it is mandatory, from the politeness standpoint, to tip the driver, since she did provide some "service" in the generally understood sense of the word.