Many foreign banks will only take the latest iteration of US currency. Thus, holding onto the old bills would create unnecessary friction. You'd have to take

Many foreign banks will only take the latest iteration of US currency. Thus, holding onto the old bills would create unnecessary friction. You'd have to take the cash to the United States to be guaranteed to exchange them for face value. But cash exceeding $10,000 has to be declared to the customs agent -- which is even more frightening. Better to exchange it right away, so you know you can use it wherever you go.