+1 for Opposite - as long as the nitro has cured before the pickguard was attached, there should not be a problem.
Find an older nitro guitar and pull off the pickguard, almost looks like it was just buffed underneath. If this were a big problem, other parts like plastic pickup rings, cavity covers... could not be used.
Manufacturers need to put warning labels on products, sometimes for valid reasons and sometimes to protect themselves (sometimes from idiots).
Here is a good example - I have my own brand of Hot Sauce. Hot Sauce does not need to be refrigerated (as long as the PH in the recipe is at the right level) but every bottle you find will say Keep Refrigerated (or something similar)...Why? Not because the sauce will go bad, but because the person putting sauce on their burger (or whatever) may touch the food with the lip of the bottle, put it back up in the cabinet, then will blame the maker for the furry growth that appears on the sauce.