An LEO should use due caution on ALL traffic stops and other investigative stops. It would seem to be good manners to inform an LEO if one is armed. Seems rather simple to me.
BTW, I am a Texas peace officer, and I was taught a long time ago that we should inform a fellow LEO any time we are armed when being detained. (This was well before Texas had a CHL system in place.) Good manners, and prevents surprises.
I don't know what the feds taught/teach their folks, but I remember an FBI agent that we had stopped on traffic, well before the CHL system existed in Texas, politely informing us that we was a federal agent, and armed, while keeping his hands on the top of the steering wheel.
FWIW, a CHL holder informing ME that he is licensed to carry, whether armed or not, makes things run smoother for both of us. I know that a CHL holder is most likely a "good guy" in a given situation, all else being equal.