Not if you don't know they're dragons.
I'm a newb to SR, of course, but not to gaming in general, and I wholeheartedly agree. Dragons can take many forms, and it can be a great surprise if someone they thought was a low-level whatever suddenly turns out to be much, much more.
But a friend and I played a D&D3e campaign for a bit at our FLGS and the FIRST adventure involved dragons in their natural form. Granted, the DM at the time was using his own home-made world, but I generally think all settings (SR included) should have a general rule: dragons are badass, use sparingly (at least in their natural form).
However, an awesome campaign could probably be built where the characters are being used by a dragon from the get-go for his own personal gain; they just don't find that out until they've earned dozens of karma. Maybe he's their fixer for the first 20 runs; then, on run 21, tries to get them killed off to cover his tracks. How do you enact revenge against a dragon? Many peopel might think that a dragon doesn't want to get his hand dirty like that, preferring to work deep in the shadows, but it's been rumored that Ghostwalker has lost some of his power. Maybe there's a reason for that...