I know things... A lot of things.
Join Date: Feb 2015
Motorcycle: GSXR 750 K4
I have no direct experience with a SET valve, as my K4 doesn't have one, nor have I had much occasion to investigate it, for the same reason. I would guess that the throttle valve itself, will be mounted on sealed bearings and require cleaning at most, as far as periodic maintenance is concerned.
Any grease, assuming it will survive the heat, would likely serve as glue for carbon particles to accumulate and be baked solid, so I'd avoid it unless suggested by the manual. Keeping the exhaust stream clean by avoiding modifications that affect the AFR, or remapping after any such modifications might also help.
As I said, I'm just guessing; others with more experience will hopefully know more and if not, go with the manual. (The latter should perhaps take precedence, depending on your outlook on things.)