A while back, I read somewhere (a book on aircraft design) that flying wing airfoils should have low pitching moments. However, as adamdb pointed out, it's not critical, and apparently no one pays much attention to that in RC. If you page though some of the airfoil libraries out there, you can find airfoils specifically designed for flying wings. But you can make a wing fly just fine with just about any airfoil (or no airfoil, eg PBF).