To add, always turn off the AC before turning off the vehicle. Never leave the AC on. Well actually it is your car so do what you like but I recommend you don't.
This is a myth (sorry Christine), very old school practice that was used back when compressors were the old 6 cylinder style, or York "V" type (very heavy mechanical load), and before cars automatically shut off all accessories during start which all modern cars do now. The small R-4 type compressors in use today are much less of a mechanical load.
When you start your car, the air conditioning and main accessories are automatically turned off, so you don’t have to manually switch off the A/C or stereo before starting the car. These accessories are a big electrical draw on the battery if they're turned on during startup, and if they didn’t turn off automatically it could be such a large draw that the car may not be able to start. Next time you start your car, pay attention to the radio and other accessories, notice that they will turn off briefly when you crank the engine to start it.
BTW, I'm a certified GM A/C tech. - Dan