The symmetry properties of the parameters of the effective dipole moment expressed as a series of vibrational and rotational ladder operators have been investigated for the case of symmetric-top molecules. It is shown that the expression for the Herman-Wallis factor obtained by Watson [J. Mol. Spectrosc. 153, 211 (1992)] for fundamental bands of symmetric top molecules could be used for overtone, combination, and difference bands in the case of no accidental resonance and if the l-type interaction is negligibly small.