No! Especially if this is GMail we're talking about, *archive* and not delete. Delete should mean GONE (with a 7- or 30-day safety window, for example).
- I do agree with the second point. Delete if you've read it, do not need to act on it - but keep it if you thing you might possibly need to refer to it someday. Archive it.
- Avoid leaving things in your inbox. Do it, defer it, or delegate it. If you defer, set a calendar event or a todo item (that links to the email message if possible) and get it out of your inbox.
Keep your inbox like your physical mailbox. You don't leave stuff like bills in your mailbox until you are ready to take care of them, right?
Basically it boils down to: don't use your inbox as your task list! Otherwise, your process list is all backwards. Whatever comes in most recently into your inbox seems to take the highest priority, and it shouldn't be that way. Use a proper task list or calendar for stuff you need to take care of.
It is a bit of work to do this, but keeping organized is not automatic, it takes some discipline.