Everything I've read says there's no reason to believe that they are hereditary. They are much more likely to occur in girls.
Despite both those things, my two boys each have one.
Me - Aimee - Spot on liver
Elliot, 9/20/06 - H on back (nearly gone)
Felix, 7/28/08 - H on forehead (in involution)
Taking the 'wait and see' approach, for now.