Our SOP is that 121.5 is to be monitored on COM 2 when not using it for speaking to the handling agent, obtaining the ATIS or listening to a VOLMET. I do turn the volume slightly down compared to the primary frequency on COM 1.
I don't have the Jeps handy at home, however, over Europe (France etc) we need to monitor 121.5, over the UK, ironically, we don't because of Mil DDD. (Please excuse me if I have the name wrong, I have met the folks a few times, great blokes with fantastic kit)