You can use them either way. For bolt guns it is not necessary to use them. For my gas guns I do use the expander ball since some of the necks are dented upon extraction. Running the expander through in the up stroke rounds out the necks. The key when using the expander ball is to lubricate the necks and use the proper size bushing to minimize runout.
"Emergencies have always been the pretext on which the safeguards of individual liberty have been eroded" - F.A. Hayek