Should I use liquid water proofing membrane over a standard shower mud pan?
Any shower I have done in the past 5 years has been done with both the PVC liner over a sloped bed and the liquid applied membrane. I coat all the corners and the drain 1 coat, then paint 2 coats over everything up to the splash line of about 5 1/2 feet. No need to do the ceiling.

If you have not done it already, let me suggest this.... I use a modified thin set and fiberglass tape to cover all the joints before applying the liquid membrane. It strengthens the joints where the cement boards meet. Also bridges gaps for the membrane.