I think top down has the huge advantage of creating consistency. Starting with the geography and culture first, everything you create after that can be explained as a result of these. If you start with one location after another, you will likely end up with some places that make lots of references to other places and cultures, while the earlier ones have none of that.
Though it does depend on the intend of the setting. If it is the society and culture of the world, that interests you, top down is almost mandatory, but not all games really are about that.