I always go where BDu tells me, following known trade routes there and back. If I'm lucky, I might not make it to the assigned grid before running out of torps due to a hapless convoy I run into. I usually stay in the grid for a week in boots of the VII class and up and then make my way home along the same routes I used to get there, doubling my chances of hitting a convoy. I don't like to travel home torpless so always keep 2 or 3 in reserve for targets of opportunity or self defense.
I also use WB's Renown Replacement so my efforts are rewarded. Find it here:
http://www.subsim.com/radioroom/down...o=file&id=1182