if you are ever in a "situation" and have an epirb in the glove box activate the thing before you go in.
What I'm missing in this discussion is the fact that even a modern 406 MHz ELT/EPIRB/PLB will still transmit a homing signal on 121.5. So if you activate it (and I strongly suggest you do so at the earliest opportunity, particularly over water), you would be stepping on your own transmissions if you were to transmit on 121.5.
Does anybody know the signal strength of your average ELT/EPIRB/PLB vs. the signal strength of your average VHF COM? Can you still send and receive on 121.5 while the ELT/EPIRB/PLB is activated?
I seem to remember one post on here from quite a while ago (personal account of the aircraft ditching in the Irish sea perhaps?), that suggested you can't. That would make a strong case for not changing to 121.5, but staying on the current frequency.