New features for Drupal 6 will be developed in this release.
This code is stable, but not well documented. And can be safely used on a production website. We will be adding new features before the official 6.x-2.0 release and possibly changing a couple features, but this -dev release will continue to work “out of the box” at all times.
So, developing a sub-theme on a particular download of 6.x-2.x-dev should be relatively pain-free. However, developing a sub-theme while constantly downloading the latest 6.x-2.x-dev may be challenging.