[Halfir]

Thingol: thanks for your very comprehensive reply. I too had inclined to the idea of the weakness of Sauron to control without the ring the capriciousness of dragons, and your very helpful post comfirms me in that belief.

Dragons quite clearly had their own ideas and pleasures - mainly to do with the acquisition of riches (in that trait they much resemble the dwarves!) Glaurung, for example, at the taking of the Elven fortress of Nagothrond turned on the Orcs who were plundering the fortress and took all the plunder for himself. Given such wilful 'servants', and without the power of the Ring, it is more than likely that Sauron was unable to coerce them to his service.

On the other hand, as was also posted, he might not have wanted to, or the dragons themselves might have decided to opt out of any future dealings with him! However, I prefer your theory.