Much like the principle behind the pre-EQ, a post-EQ serves more of a purpose in terms of controlling the entire frequency spectrum as multiband compression can often extract and enhance some unwanted sounds. The post-EQ will typically be un-bypassed and used to attenuate any surplus of frequencies further balancing the spectrum prior to the saturation and loudness stages to commence.
Rarely will a good master need a boost in a post-EQ, typically, attenuation is the only task to be completed. If you feel you need to boost the EQ here, you may need to revisit the prior mastering stages or the mix itself.
Q values can be fully subjective unlike the recommendation for tight Q values in the low end and wide Q values in the top end for the pre-EQ stage. Bear in mind, tight cue values can create irritating resonances while wide values will maintain the overall mix with less transparency. Use up to 1dB for attenuation to begin with and revisit if needed.
For the upcoming stages, a surplus of low frequency energy will trigger saturation and limiting far more aggressively, use this EQ to compensate for any after-effects.