In my application, I want to create entries in certain tables when a new user signs up. For instance, I want to create a userprofile which will then reference their company
Simple solution, add this to the beginning of your post_save function:
if kwargs.get('raw', False): return False
This will cause this function to exit when loading a fixture.
See: https://docs.djangoproject.com/en/dev/ref/signals/#post-save