You dont have to use the 12v trigger,as you can set the sub power mode to "auto" and the sub will remain in stanby until it senses a input signal is being received via the rca connection.
However,MA recommend using the 12v trigger as a more accurate and energy efficient means of control.
In fact MA say....
"For external power control from AV amplifier/ receiver to the RXW-12. Monitor Audio recommend using this function and a lead is supplied. This 12 volt signal is supplied by the AV amp/receiver and tells the RXW-12 to turn on from standby mode. This allows a more accurately controlled auto on/off function, and is far more energy efficient. When using the 12v trigger, the Power Mode Switch (5) can be in any of the three positions, for it to function correctly"
(from page 6 of the manual)