All damage is unnecessary. You are not going to incur damage by operating within limitations. You will not achieve the maximum life from the engine, however, and in turbine engines, a small reduction in ITT/EGT can provide a substantial increase in service life and or a significant reduction in wear. Heat in the turbine is the enemy.

When I run a PT6A-67, I run it at the max continuous EGT as long as I need it there, which is during the extended climb, because that's what the engine is rated to do for that period of time. Derating is irrelevant.

Derating the engine allows the engine to be operated at it's maximum rated shaft horsepower to higher altitudes, rather than running into limitations early on and having a torque or temp curve to respect.

I don't operate a PC-12, but I do operate the -67 quite a bit. The max continuous ITT for our operation is 800, which I'll use until in cruise, and reduce to whatever value is needed to run at the top of the green/bottom of the yellow. It's typically in the 750 range, torque in the 3000 ft/lb range. I'll generally leave the prop at max 1700 if I'm heavy, and wont' retard it until I'm empty in cruise, then back to 1,500. We're derated substantially. I run up against torque limits at lower elevations, temp limits at higher elevations, as one does with a turboprop.

I don't have limitations other than max for the aircraft I'm flying, but in any aircraft, I observe the most restrictive of the engine or airframe manufacturer limitations.