I think I found the answer in NetSuite's documentation, in case you are curious. They say "If you are using the Foreign Currency Management feature for Multi-Book Accounting, the base currency for the accounting book must be defined for each associated subsidiary." That leads me to believe that in our US book, each subsidiary should have USD as the base currency selected. That is not what we have. Instead we have the subsidiary's local currency selected as the base currency. So we have like 11 different "base currencies."