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I believe that immediately after 9/11/01, there was an FDC NOTAM (one of the many published) that mentioned using TN for charter operators, but I don't recall the exact phrasology (should vs. shall, etc). As FDC NOTAMS are regulatory, there seemed to be a short-lived increase in the use of TN. Since then, I haven't seen it mentioned except in the AIM.
At my old fractional, we used to use it when flying internationally, as our callsign was not recognized outside of the US ATC system.
I used to use it frequently on 135 flights (if I was the one to file the flightplan), but most people do not. Never heard about anyone getting in trouble for not using it. I can't imagine why ATC would care whether we're operating 91 or 135. Still, if it's in the AIM....