[marcia]

David Allen's filing system for reference material refers to paper files needing a simplified system for manual search, and the reference files themselves serve a different purpose than task management. So I don't think it's necessary to follow a shallow filing system based on that unless you really want to. It's more a personal choice.

At the top level in OF I have four folders (Actions, Projects, Goals, Checklists), and one project (Someday/Maybe). Some of the folders contain other folders in addition to projects.

This is for the purpose of managing my sidebar so it doesn't appear cluttered.

Even with a hierarchy of folders, OF can give a flattened view of all the folders by selecting Folders under the Grouping filter. For example, I have a perspective called Next Actions that groups everything by folder and shows the next action for each project. All the folders are presented at the same level giving you a bird's eye view.