How do you share a sandbox?

tkp's new War Stories 'verse got me to thinking and that's really no surprise because tkp inspires most of my thoughts these days and that totally makes sense considering the shininess of her brain.

I know there's been discussion on this in other places, and if anyone remembers the link please forward it. How do you share a fandom created universe? Or do you? Can you? Is it possible to play in someone else's sand box without stepping on anyone's toes? Do you ask permission before you write, or before you post? Do you ask permission at all? Does the fact that fandom is predicated on appropriation eliminate the necessity to ask permission? Is it assumed that all is open to everyone or ... well, what's the deal?

( Universe Appropriation Among Fic Writers )