A comment here suggests that
Whenever you kill a boss for the first time (on normal difficulty only), it is guaranteed to drop at least 2 rares: One general rare and one specifically suited for your class
Is this true?
I'm fairly sure I didn't get anything but blues the first two times I killed the Skeleton King (although my friends got yellows), however the Spider Queen, the Butcher, and Maghda all dropped me two yellows. I can't remember if one was class-specific in all cases, however it was true in at least some cases.
Edit: I've verified that the Skeleton King does not always drop two yellow items, and am trying to determine if A) this quote is correct, incorrect, or partially incorrect and B) if correct or partially correct, does this only apply to end-of-act bosses?