When flying on a plane with two or more radios, should I set the second radio to 121.5 at all times? (Assume I'm not using it to listen on other frequencies)
I've heard that sometimes when an intruder penetrates restricted airspace, the military will broadcast on this frequency in an attempt to communicate with that aircraft. Is this a good way to avoid me getting shot?
What about monitoring in case someone around me has an emergency?
Is there a difference in general aviation vs commercial airlines?
EDIT: I'm asking about whether this is a good idea, not about regulations.