It all depends on what you are doing with the meat.
If you are using ground beef, then it's okay because it will just fall apart. I cook frozen chicken if I am making soup or if I want to chop it in the end for a meal.
Otherwise, if you want to cook a chicken breast, a beef roast, etc, I would let it thaw first so you get more even cooking. You can thaw it pretty quickly by letting it sit in a sink of cold water. It usually works like a charm.
I hope I answered your question.