I would probably say Leon is correct, that the best time to degrease the soup would be when the soup is boiling. Now we have to remember that grease pretty much is like oil so what would happen to the grease when it turns cold? It would become harder and would probably stick itself to the soup already. So with that said, it would be easier to degrease the soup while it is still boiling because it is still not embedded to the soup.
