Having personally operated the first commercial PT-6A powered airplane on the US west coast, circa 1966, ice deflector vanes should be used only when absolutely needed.
To do otherwise is poor advice.
Having been the director of maintenance for a King Air operation using both the 90 and 200 series airplanes, as well as an instructor and line pilot in the same, as well as having operated the PT6 in fractional, agricultural, firefighting, and other operations...you're quite wrong. Of course, one needn't be the first to operate the airplane, or hold a management or line position in order to read the aircraft flight manual and understand it.
The ice vanes are inertial separators intended for use not only in ice, but in any ground operating conditions where engine contamination and FOD is a threat. Furthermore, if the aircraft is equipped with Raisebeck ice vanes, there's no loss in power with their use, and they may be used continuously, as required.
The thread asks about PT6 operation, and the ice vanes are a customer option, not a Pt6 installation; they're actually airframe (nacelle) installations, and not actually directly part of the PT6.
Actual ice protection is provided by either an electrical boot or a bleed heated inlet lip, depending on the model. Ice vanes do not prevent icing, but simply deflect inlet contamination from ice removed from the inlet area, as well as preventing fodding of the engine by gravel, dust, or other contaminates.
Ice vanes should be extended in visible moisture below +5 degrees C, or if in doubt as to icing potential or presence...per the aircraft flight manual. Ice vanes should also be extended when inertial separation is needed, which may include operations from loose chips or gravel coated runway, or airborne contamination such as flying in ash or other contamination.
As concerning windscreen anti-icing heat...only necessary where the AFM recommends same, and....with NESA heated windshields, especially....referenced, F.27/FH227, DC6, DC7, Lockheed Constellation, Boeing Stratocruiser, CV340/440, (etc) aircraft.
Many aircraft with NESA windscreens recommend their use for bird protection at lower altitudes...but then what a Convair, Douglas, Lockheed, Boeing, or Fokker recommend or do really has nothing to do with what one should do in a King Air. Again, posting one's resume does very little to address the direction provided by the aircraft flight manual. You're correct that one should observe the aircraft manufacturer recommendations. In this case, Raytheon (Beechcraft) procedures...not Convair, Douglas, Lockheed, or Boeing.
You're certainly better off having the heat on than waiting until the windshield is cold soaked and turning it on.