Mock ups have not been something I would always do prior to starting (or painting) a faceup. Now it is a crucial part of my process, whether they are asked for or not. I have grown to depend on them and even enjoy doing them. It's a way to experiment without wasting supplies or becoming frustrated. I highly recommend doing it if you are doing faceups, it will make doing one a lot easier because you have a reference to draw from. The Oasisdoll I just recently finished had a mock up created before I began painting. You can see how I was able to experiment with the lid lines and then transferred that style onto the actual faceup.
I use Photoshop CS4 and a wacom tablet to do the mock ups. So I have a much easier time creating similar lines on the computer as I do with my paintbrush on the doll. I have done some mock ups that are my visions for a client's dolls that I want to get their opinion on before I start. They may be accepted, tweaked, or completely dismissed, but it's definitely an easier way to work. We'll see if these guys will be brought to life.