The U.S. supply of munitions to Britain was definitely "sketch" prior to official DoW in December 1941. As far as I'm concerned, at least following passage of Lend-Lease in March 1941, American shipping en route to Britain should be fair game with normal renown points awarded.
But that still leaves U.S. shipping during the "Cash & Carry" period of early war, i.e., 11/39 to 3/41. According to U.S. law prior to Lend-Lease, American civilians were not permitted in "war zones," as defined by Roosevelt's office. The "neutrality zone" originally consisted of 200 miles (~ 320 km) off the U.S. coast but expanded in 1940 to 300 miles (~480 km). Hitler extended the German definition of the "War Zone" in the Atlantic to the east coast of Greenland in 1940. So the whole thing was very sketch, as Roosevelt wanted.
But it is not for us, mere kaleuns, to question the Fuhrer's strict and clear directives in 1939 and repeated through mid-1940 to the Kriegsmarine forbidding U-boat attacks on American shipping in the Atlantic. While Roosevelt was scheming to fight an undeclared hot war in the Atlantic, Germany did not wish to repeat the mistakes of the Great War and mobilize American public opinion to intervention. We must assume that our Leader would greatly frown on sinking of American shipping, at least prior to May 1940 or so.
A very key date is 8/17/1940, when Germany declared a total blockade of Britain and formally warned off all neutral shipping and air travel. Using this date is trickier if you're going to modify the .cfg file because you're getting into geographically-defined belligerence/neutrality.
Maybe easier to stick with timeline-based tweaks to the .cfg file! -1 penalty until May 1940, then 0, then +1 after March 1941.
