Why didn't Sauron make use of the dragons?

[Inderjit Sanghera]

In FotR, Gandalf tells Frodo that there are no dragons left that could melt Rings of Power. This suggests two things. There were dragons left after Smaug, the Great Dragon, but, they were not useful to Sauron. Perhaps also, these poor excuses for dragons lived in areas uninhabited by Sauron's enemies and could not be tempted to move.
--Imladrien

I think he said that no Dragon could EVER burn the ring of power, except for maybe Ancalagon, though he was doubtful about him, too.

I think that there is a mention of Dragons living in the Northern Waste, close to Ered Mithrin (The grey Mountains.)

I also found this quote form the tale of years;HoME 11:

"Ancalagon is cast down by Earendil and all save two of the dragons are destroyed"

As i have mentioned earlier, it may have just been a brief note, soon abandoned by Tolkien, on how many Dragons escaped the War of Wrath.