The effect of valence subband-mixing on the optical properties of strained and unstrained quantum wells (QWs) is analyzed theoretically. In the model developed, the orientation of the confined carrier in-plane wave vector and the optical polarization vector, which are taken into consideration to obtain the optical matrix elements, are believed to have a negligible effect and have been ignored in most previous calculations. Results presented here show that there are large variations, due to subband mixing in the of compressive strained QW, such as InGaAs/GaAs, and which addresses the controversial issue of whether a non-mixing calculation, such as the parabolic band structure approximation, should be used for strained material systems. The effect of orientation also shows an over-estimation and under-estimation of the TE absorption coefficient when non-parabolic and parabolic band structures are used, respectively, under isotropic orientation. These results indicate the importance of an accurate model for the determination of optical properties in the QW structures.