I voted 'It Depends.'
When I write my sim posts (and the single, solitary badfic fanfic I've written) I usually have a starting point, and the story just progresses from there. In the case of simming, there's really no end in sight, unless the sim shuts down.
On the other hand, when I've had to write something on a specific topic, like an essay or paper or anything without a story to it, I usually would write the beginning and the end, then flesh out from it.
