They are not always needed for all modes of flight. This is proven by both some full sized versions along with a few models. However while Northrop and Hortten didn't use any on their bomber or sailplane flying wings it's interesting to note that all combat aircraft flying wings DO use some form of fin. I suspect it's because the simple wing shape is only stable in yaw over a limited range of aerobatics..... much as kingwoodbarney found.

Swept wings will provide a very limited yaw stability due to simple drag. As the model yaws more of the leading wing is presented to the airflow and that should make it pull back into place. But I'll suggest to you that it's a weak force that can easily be overcome by other factors.