I always leave my second radio on 121.5 . This was a recommendation of NAS however all the education stopped when the frequency boundaries were put on the charts as no one could work out how such a half wound back system could work.
From experience the best frequency to call on in an emergency is 121.5 as all high flying airline aircraft monitor and record this frequency. Even half way across the Indian Ocean I have had an immediate answer from a test call on 121.5.
In the USA it is a requirement for radio equipped aircraft to monitor 121.5. Obviously only where practicable. It's a sensible requirement as ATC can then get another aircraft to call on 121.5 if an aircraft has entered restricted airspace . The US requirement came in after September the 11th.