As I understand it, the cocoa-rich type of chocolate can be fatal to dogs, even in very small quantities.
The rule of thumb used to be no contintental choccie for dogs, British (Cadburys and the like) would be okay - partly because the cocoa content would be so small as to be insignificant. But these days, quite a lot of chocolate has a much higher cocoa content, so I would be very very careful.
Clearly, where a dog has been enjoying a certain brand all of its long and healthy life, there isn't much to worry about - so long as the manufacturers haven't changed the recipe too much...