Allan,

Most of the calculations start from assumptions and they involve compromises.
Being no rocket science, the estimation of a safe location for the CG previous to a first flight may be in error, but is always better than a non-calculated or a subjective guess.

As long as there is enough authority of the surface that balances the main wing(s), the CG can be located at a safe or exaggerated distance ahead of the aerodynamic center for those first flights.
The shading of one surface with other may be a myth, but it introduces harmless error toward the safe side.
Later on, when any other problems are solved, we can slide the CG toward its limit and test until the plane and us are both happy.

Tail heaviness, however, is not a myth.
It is an unstable condition far from which you want to have your plane at least for its maiden flight.