I try to keep a balance of fore and aft, but actually prefer to use aft tubes if I can get my boat into position. Attacking from aft helps me slip away if merchants are escorted, or finish them off quickly with my deck gun.
If I have to attack an escorted convoy using Forward Tubes, I immediatley set my aft torps to high speed at shallow depth. As soon as I launch the forward tubes, I turn in the direction the convoy was heading and evaluate the intentions of the closest escort. If he even looks at me crosseyed, I launch decoys so he comes running, and then send an aft torpedo to greet him.
My favorite tactic is to place my sub directly in the path of a large convoy, and then pop up to periscope depth when they are right over the top of me. I use both fore and aft tubes simultaneously - engaging 10 targets and running for the thermal layer. This places most of my targets with 2,000 yards without much room to move. The remnants of the convoy inhibit the escorts ability as well. Just make sure one of your targets doesn't start sinking on top of you. Nearly got sunk when a tanker collided with my boat on its way down.