Reliability of new 2015 6.7 powerstroke or Cummins Diesel

"Use all the time, or it will carbon up.

Is that a real fact, or myth?"

It's a myth. VVT's go through the same motions everytime you punch the skinny pedal as they do when the e brake works, except in reverse.
Using the exh brake will not do any more than just driving it to prevent the turbo vanes sticking.

Now that is true of butterfly valve exhaust brakes. If they remain static for alot of hours/miles, they could cake up and stick theoretically.