That's a bad plan.
Turn the volume down. Your sound quality sucks without your subs on anyway.
The amp turns off when the radio turns off.
If you want to listen to music with the car off, and you don't want to set it up with a battery in the trunk so you can keep the amp on, then yeah, go for the switch. But seriously, how much do you sit in the car listening to music for more than an hour?