B738
In my last company the APU was turned off when we had finished with it. (Once engines started and generators on bus etc.) Then turned on for when we needed it, i.e. while taxiing in to provide power to the A/C once the engines are shut down. If we were doing a Low Vis App it would generally be turned on passing FL100 in the decent because of needing another generator supply.
Just what we did.....