Deterministic sources of polarization entangled photon pairs on demand are considered as important building block for quantum communication technology. It has been demonstrated that semiconductor quantum dots (QDs), exhibiting a sufficiently small excitonic fine structure splitting (FSS) can be used as triggered, on-chip sources of polarization entangled photon pairs. As-grown QDs usually do not exhibit the required values of the FSS, making the availability of post-growth tuning techniques highly desired. This article reviews the effect of different post-growth treatments and external fields on the FSS such as thermal annealing, magnetic fields, the optical Stark effect, electric fields and anisotropic stress. As a consequence of the tuning of the FSS for some tuning techniques a rotation of the polarization of the emitted light is observed. The joint modification of polarization orientation and FSS can be described by an anticrossing of the bright excitonic states.