Like others have said, it can be anywhere. On some military planes it used to be a point far forward of the nose...not even on the airplane itself. The exact location was a secret, this way the W&B sheets did not have to be classified until filled out.

It doesn't matter where it is, except I suppose that on a non-swept wing airplane with symetric wing tanks you could locate it at the center of the fuel tanks. This would simplify calculations as any changes in fuel would require only a weight change, as the arm would be zero.