I assume that you don't want to automagically post the form on every keystroke within a field, since responsiveness would become decidedly unfriendly.

If that assumption is true, you need to trigger on transition out of a field (or into another one). That opens you up to the edge case of no transition, such as the user typing into a field, then doing something like hitting the "Back" button or typing in a URL. There's no onClick event to capture to trigger you examining fields for changes. Would a resonable user believe that the database should have changed in a case like this?

The nice thing about an explicit "Submit" is that it's unambiguous, with no edge cases.