Declaring derived class destructor virtual

I know it is only necessary to declare a method as virtual in the base class, and the overridden version in the derived class will automatically be virtual.

Does this apply to destructors as well? In other words, is it necessary to declare the destructors of all non-leaf classes in an inheritance hierarchy virtual, or just the base class?

Yes, it does, so declaring the derived class destructor virtual is unnecessary, but good practice.