I once was an Inbox Zero fiend, but I’ve long since tempered that. This comment sums up my current attitude towards mail handling:
I don't follow any particular fad, but I do try to keep my inboxes empty—if I leave a read message in an inbox, it's because the message is critical and I *want* to be annoyed by it so that I don't forget (e.g. bill due today, meeting reminder, etc.).
In related news, I’ve installed this plugin to Mail.app to save myself some time when doing interleaved replies.