how far should I get with the tests in tdd?

My question is how far should I get with tdd testing? For example: If I have a class A, which uses a class B and C to work, and these in turn use other classes D, E, and F, etc. And I have an X class, which uses Y and Z to work. If I wrote the tests for A and X, and I have 100% code coverage, is it necessary for me to test the rest of the classes if they are already covered? If my code is coherent, the tests should remain close to the consumer classes, or am I wrong?