I use Photoshop SR4 and find that the Exposure correction layer is a great way to get the overall effect I want. For example, to avoid washed out areas I usually render with a little less light than the final image will have and then boost Exposure by 0.2 or 0.3 which adds brightness while maintaining color saturation. Then I use the gamma correction if the shadow areas are too dark.

Some people add backgrouns in postwork. I use a background prop like the one by Emma&Jordi because I can position and scale it to get exactly the result I want in my render.

My most common use of postwork is to fix little glitches in 3D and capture them in partial renders which I overlay in postwork.