My rig is a type VIIC in mid-'43. I always sail at Ahead Slow to conserve fuel. I also believe my watch crew have a better chance of spotting the enemy while I'm at AS. Furthermore, my sub doesn't produce such a large wake for aircraft to spot (the advent of radar reduces this advantage from 1942 onwards).
At the beginning of my career I usually journeyed at Ahead Standard, but that left me with too little fuel to chase more than a handful of targets. Ran out of fuel once, and didn't want to repeat the embarassment.
Recently, more and more convoys do around 13 knots, meaning I have to crank out a lot of knots over 10+ hour periods to do an end-around. Conserving my fuel pays off.
On homeward journeys, i.e. when all my ordnance is gone, I often travel at higher speeds since I no longer need to conserve my fuel.
Beginning somewhere in 1942 I order PD at dawn and surface at night. It helps preserve fuel as well, but that is merely a secondary benefit.
Kind regards,
Eichenlaub