Does anyone notice a loss in power when the air con is on? I remember in the old mk 4's when you had it on it felt like it used half the engine power 
This is the main reason to turn it off. My MK 4 2.0 (I still have it) has a noticeable power loss if it's hot and the aircon is working hard. Not so noticeable on the MK 7 but it must consume a few KW of power.
If you're cruising no problem leave it on but when the tailgating BMW appears in the rear view mirror turn it off until he's gone away.