I always saw it as a throwaway expression that wasn't meant to have a literal meaning... like saying "bob's your uncle"... but it was picked up on by some fans, and support for it being real has snowballed it into canon.
I've always assumed it was cannon; Ever since hearing it as a kid, I never even considered the thought of it being a throw away line like 'Bob's your Uncle'...
(Now I'm scared! )