With virtual destructors, do I need to explicitly declare a virtual destructor for each subclass?

The default destructor is not virtual. If you declare the destructor of your base class as virtual, the destructors of the subclasses will be overrides, and thus also be virtual even without explicitly declaring them to be.

The GNU GCC compiler even gives a warning if you have a class hierarchy and your base class does not declare the destructor to be virtual because you most likely want it to be.

The answer is no. The only relevant requirement here is that classes with a vtable (i.e., with at least one virtual function) must have at least one a virtual destructor somewhere in their inheritance chain. Typically this means that your fundamental base class will provide an empty virtual destructor.

In general if some function is declared virtual in base class, there is no need to explicitly declare it virtual in subclasses. However it is good practice.

Declaring destructors in subclasses as virtual explicitly doesn't give you any serious advantages, so if you don't wont to write one more virtual, don't do that.