Great feedback here
In RL, high priority targets (Tankers, Troopships, Carriers, BB's) were not to be ignored, especially if a favourable firing position could be obtained.
In game, I'd certainly welcome the opportunity and sink the higher tonnage ships.
3 to 4 torpedoes left in RL, a U-boat would usually radio to announce it's arrival at the deesignated patrol area. Most definitely in extenuating circumstances (and I'd contend being off the east coast of US with only a handful of eels left as being a prime example).
It was not uncommon for a IX returning from a mission without all it's ordnance and fuel spent to meet up with an 'incomer' and pass over a little fuel or the odd eel.
In game, I'd head for a milch cow then proceed to the original grid after replenishment.
If your playing GWX2.1 you won't get any renown for the patrol grid either way anyway.