Sinking that sea truck with your deck gun might mean the expenditure of much needed rounds later
I always save my war-fighting capability until I've reached my patrol area, unless I run into an unmistakeably high-value target. I consider aircraft carriers, battleships, oil tankers, troop carriers and enemy submarines to be up for grabs; convoys bound for England, too. This assumes that I actually run into anything other than patrols, of course
Even though the game doesn't really take strategic tracking of your Boat into account, I always prefer to avoid detection and slink stealthily into my area before I kick up a hornet's nest. I might even give the area a couple days to cool off after a particularly hard-hitting go of it.