I play good soldier and follow orders-go to my patrol grid and patrol for 24 hours. On the way to my area, I'll go after reported convoys/contacts if I can easily intercept.
After my 24 hours is up I'll freelance a little bit and within reason-I'm not going to sail to the other side of the Atlantic to a favorite hunting ground.
If I'm close to an area where some historical event is going to take place(Invasion of Norway, D-day, etc.) I may position myself to take advantage of it on the assumption Bdu ordered me there.
I'm going to try out Sailor Steve's idea of rolling a die to introduce some randomness on whether I stay in an area or not. I don't need or necessarily want 60000 ton patrols every time by going to a favorite hunting ground every patrol. Plus, I'm in late 1943 with a couple of careers and I'm pretty happy with a few ships sunk and getting home with my hide intact.