I am far from being an expert at this, but I was under the impression that your body knows when bub is born, and therefore knows to produce colostrum. For example, women who BF during pg still produce colostrum when the new baby is born. Please correct me if I'm wrong though.

DB, I agree with the PP about the stimulation not working until your body is ready. And doing it manually is probably more effective. Michael is probably right though, it likely won't hurt, you just may not get alot of benefit. And it won't hurt to become familiar with your breastpump, and how it all works. And if you do manage to express anything, you can freeze it for when bub arrives