Depends on the rules. I have a habit of clarifying these at briefing. 'What's team A's channel, what's team B's, which is marshal channel in case of emergency.' Listening in after being told the channel to avoid is a dick move. Not knowing the marshal channel is daft in case someone breaks a leg or similar (again).
Picking up an unattended enemy radio and sending false info, listening in etc is 100% fine in my book. It's also great fun to use a stupid callsign and then mention what you did around the campfire later that evening...
I only play filmsim though.