“As a rule of thumb, make all your methods virtual” in C++ - sound advice?

Question

I just happened upon the statement in the title. The full quote is:

As a rule of thumb, make all your methods virtual (including the destructor, but

Answer 1

There will be a tiny loss in performance and a few bytes of memory wasted.

The real problem is that it makes the code less maintainable because you are saying something about the function that isn't true. It could cause a lot of confusion.