I'm curious to know how many here use some kind of origin story to help shape their world. My initial attempts at world building in the nineties never had a core origin story and I think that the lack of one had a negative impact one my world design. I'm about to embark on my first world building project in quite some time and this time I have a very specific origin story, a definite plan of how my world sprang into existence, and it's really going to help me define how my world looks. My origin story will have a very real impact on the geography and, in many aspects, the culture of my world.
Do you go so far as to come up with an origin story for any of your worlds? Is it an absolute must for you or something that would just get in the way of your world design?