I use them on items that contain rare mats that are pricey at the trader, that way I don't have to buy them for armor. Or for rare mats that are easy to sell to other players, like rubies, monstrous claws, etc.
99% of the time i use them on the gold great conches from raptor farming during events. with a moderate farm rate of 4-5 hours a day, during a weekend event i can get 20-25 stacks of dust.
i use them on anything that gives me dust,fibers(brair weapons),gems like ruby/sapphire/diamonds,or granite. other wise i save them for a weapon i get that does this for me. gold ones give more then grape then blue.