Is turning off water to outside faucet at 20 degrees okay?

I have an out door faucet which is fed from a pipe in my basement. The basement temperature is never less than 70 degrees. The pipe to the outside goes through a concrete block wall to the outside where the faucet is attached. I have heard that I need to turn off the water to the faucet and drain the pipe when the outdoor temperature reaches 20 degrees. Is that a good "rule of thumb"?
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