Would you want to know?
That's a tricky one. The problem is, if you sign up for a straight fantasy, and then a twist turns it post-apocalyptic, you might lose players.
Lesser twists - like you're working with the wrong guys, can be done, but that kind of major twist is probably best done with people you know.
As to the above point of building a character specifically for a specific story - well, with Pathfinder and a lot of other structured games, that's the assumption. Pathfinder is the obvious one with the players' guide, but most structured campaigns you will usually craft a character that works - it really sucks to build a mounted sjpecialist only to find the whole adventure is taking place in a dungeon. Now, this can work a lot better if you're homebrewing the entire campaign, because you can work around that kind of thing, but I think if you're using pre-created modules, you need to let the players know what kind of characters will work, unless you're running a system where that kind of thing just won't make a major difference.