I also served in the Navy in the Early 80's and when on Deployment we UNREPed every 3 days, the Fuel was not allowed to go below 80% capacity. the only exception to this was when a Ship was assigned to some detached duty away from the Battle Group.
As far as USN subs UNREPing in WW2, this was not as a rule done, as the Subs generally observed Radio silence to avoid giving their positions to the enemy and also as the Subs were usually in very forward areas, it would have needlessly endangered the supply vessels.
