This is the variant I currently follow: Kourosh Dini’s variation
and I actually don’t have a flagged project or project view. I wind up in the ‘treading water’ view mostly except during the periodic reviews.
I also combine this with “Inbox Zero” which in 4 words is: all email is either deleted, delegated, archived, or done [immediately]. I refile mail as _toanswer (for do later), _waiting (for delegated explicit or implicit), and _archive (for archived) folders. Stuff in _toanswer and _waiting needs to be periodically synced into GTD. That violates the only one collection area philosophy, but you can think of it as a staging area until it gets into OmniFocus.