Make implementation of virtuals also virtual?

Question

When implementing a pure virtual function in C++, is there a best-practices guideline that says the implementation should also be made virtual? What is the rationale?

class Interface
{
  public:
    virtual void foobar() = 0;
};

class Concrete
    : public Interface
{
  public:
    virtual void foobar();
};

Answer 1

It does not matter.

void foobar() in Concrete is virtual regardless whether you declare it as such and it overrides the void foobar() in Interface.