The points made about calling so everyone else has an idea of where you are and where to look if you force land/crash are good ones, as far as Chilli monsters point goes for changing to 121.5 goes, this is NOT a good idea, when things start to go wrong you need EVERYTHING on your side, your brain starts to become overloaded and changing freq. can become harder than you would believe (read your human perf. lately).
If you are already on a good freq. this not only saves switching over, but if you are short of time/brainpower and don't get all the info out the controller already has an idea of who/where you are.