Quote - Any rules on when you should use hip vs. body for rotations and translations?

Yes there are some principles. Below is the system I use when saving poses. If you don't intend to save a pose you can be a little more relaxed about translating the hip.

Before starting to create a pose, make sure that yTran for the Body and x and z rotate are zero, you may want to set all the translations and rotations of the Body to zero before you start, but that is not essential.

Don't use xTran or zTran for the hip, keep them at zero (at least in the final pose). If you use IK, you may need to translate the hip to get the desired effect, but afterwards, tun IK off and zero x and z tran. On the other hand do use yTran in the hip to set the feet - or the lowest part of the figure in the particular pose - on the ground.

The advantage of this system is that you can apply saved poses without the figure jumping to a different location in the scene, as would happen if you used x or z translations of the hip.

Pose data for the Body is by default not saved in a pose file, Poser 7 introduced an option to save Body data, but you should not use that unless you have a specific reason for doing so, and understand the effect it will have.

Use the Body translations to position your figure as appropriate in a particular  scene. Use the BODY yRotate to set the orientation of the figure for the particular scene. You should not normally use any of the other rotations in the Body.

To sum up.
 
In the hip:
Use only yTran and any rotation.

In the Body:
Use only yRotate and any translation.
(and remember, Body data does not get saved to a pose)
These are rules of thumb, not laws of nature. They make saving any applying poses easier, and prevent some potential problems. There can be exceptions. For example, if you make poses for two figures, where the figures need to stand in a particular relationship to each other, use hip translations in one of the the figures to achieve the desired separation.

Always turn IK off before saving a pose. Check the pose after turning IK off, because things can change. In particular check the altitude of the feet in relation to the ground.