The model of Mopsik and Broadhurst for piezoelectricity in polymer electrets is reconsidered and further developed. The main features of the model are reviewed with particular attention directed to tensorial aspects of the piezoelectric effect. A characteristic difference is pointed out in the patterns of piezoelectric response attributed to two distinct sources: (1) the strain dependent residual polarization associated with the alignment of electric dipoles, and (2) the presence of trapped charge injected during the poling process. Trapped charge contributes to the piezoelectric shear stress constants e15, e24 whereas the oriented dipoles do not (unless electrostrictive anisotropy is important).