You should consider how many schema parts you have.
Maybe you'll want different directories for different namespaces.
But your directory structure should be similar to how you intend to:
distribute you schemata
use them in tools
use them in applications.
Changing 'schemaLocation' attributes on 'import' and 'include' elements can be a real pain!
Changing 'schemaLocation' attributes will probably introduce bugs and so orchestrate things so you don't have to do that.