In my (very) humble opinion.....
If you need to say something on 121.5 for one of its intended uses, do so.
If you don't, then say nothing at all.
A guard policeman barking that ubiqutous 'You're on guard' blocks as much airtime and causes as much annoyance as any misplaced handling agent call, ride report, baseball score, local dialect conversation, burp, fart etc etc.....
T0ssers, the above, one and all.