Company says to start the APU 10 prior and shut it down as soon as external power is connected. That will lead to cabin temperatures north of 32C even when it's comfortably in the low to mid 20's outside.
Start the APU when boarding starts. Shut it down when everyone's off. In An A320, the cockpit can be at a comfortable temperature without the APU, while the back of the plane is hot and stuffy, so don't wait until YOU feel uncomfortable. By that time, the poor folks in the back are dying.
If the APU is inop, you can always refuse the plane. In lieu of that, you can run the packs off the HP air.