Using Smart Folders, you can also create a 'folder' that contains all notes having no tags whatsoever.
I have 4 of these folders, Untagged Today, Untagged Week, Untagged Month and Untagged.
My goal is to have them all at a count of zero notes. Which I have reached by cheating ( introducing the tag 'notags' if there isn't any subject I could think of to put it under).