There is no need to apply AO to everything; this is a judgement call and personal taste.  However, on LaFemme and L'Homme, if you exclude the face and eyebrows, the rest of the body will become mismatched.  Clothing and hair can hide the prominent issues, but you will see a difference upon bright lighting and closeups.

The gamma technique is something that can be applied; I do have a set up for Poser 13 using the gamma node.  I haven't used it much, but I do need to refine it better.

You can apply AO to hair, just decrease the distance on the Ambient Occlusion Node to reduce its effect.  Closeup portraits may present some issues, but it should be fine distant images.

The colorramp node is a new feature in Poser 13, so, unfortunately, you have to hack your settings in Poser 12.  However, the RGBramp node is very buggy ATM.  Extreme settings causes color shifts within the node negating its effect.