Noone knows what "most people" do. But I could tell you what I do.
I just set the spread angle on the TDC screen, usually 1-2 deg, 3 deg at most, never more than 2 torpedoes at once (even if I am to fire all of them, because of sudden bouyancy change problem). And I shoot. I always take I-III and II-IV pairs, as I believe this will shoot one torpedo per each side of the boat (steer and back).