Excuse my ignorance here with CAA requirements for Public Transport helicopters, but I would assume that dual VHFs would have been one of them, so there shouldn't be a problem monitoring the emergency frequency.
When we were training we were taught that it was a good idea to monitor 121.5, as it is the frequency over which ELTs broadcast, especially when flying cross country and over hostile terrain.
Even when flying tours I have 121.5 selected on back up.
Another good reason to do so is that if your machine is fitted with an ELT, on shut down tune in to see if the G switch hasn't been inadvertantly activated - it will save valuable SAR resources from being wasted and you a red face when they show up at your hangar.