Until my current game I have used dragons rarely, I can only think of one encounter (a wyrmling black in 2e). In my current game I've stuffed the setting with dragons (1 red, 1 green, 1 black, 1 blue, 1 gold, and a nest of wyrmling whites). The PCs have seen the red and the blue (but they didn't know that at the time) and know where the black and the whites lair. They have a vague idea where the green lives (with the elves), but have no knowledge about the gold dragon (except his name, and they think he's an elf)
KDM27