Eith you need a 12v cutoff both on your sub and on your receiver, or an auto setting on your sub. That said, theres no harm done leaving it on all the time. Its not really using any energy when nothing is being played through it.
You can also pick up one of the various power strips that switch off outlets slaved to the master outlet. Amazon lists a number of them, and I am sure Parts Express and other vendors carry them. APC also makes some of their Back-Ups UPS units that have switched outlets (look for their "green" units).
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley