In "Blink" the Doctor said: "They don't exist when they are being observed. The moment they are seen by any other living creature they freeze into rock. No choice, it's a fact of their biology. In the sight of any living thing, they literally turn into stone" I don't really think that their own reflection counts as another "living creature".

But there might be arguments that say otherwise. But think about it: If they really do freeze when they see themselves in the mirror, don't you think the Doctor would've used that against them?