If you don't do it already, MTB riding will make you a far more complete cyclist and therefore a better crosser. It will teach you techniques and lines that you would never learn if you just rode in circles at the local park on your cross bike. Now, there is a caveat: a lot of mountain bikers are more interested at hanging out at the rock drops and jawing than in actually covering miles. That sort of mountain biking won't help you much. But it can be a lot of fun.
Riding mtb trails on the cross bike is a good workout, but still won't teach you the skills you get from honest-to-goodness mtb riding.