You don’t need to clear out the draft styles, even after publishing. Only one stylesheet is ever loaded at a time: draft-styles.css or active-styles.css.
The draft-styles.css stylesheet only loads for logged-in administrators – when viewing the site outside of Microthemer. It always contains the latest changes you made in MT (which may or may not have been published to active-styles.css).
The active-styles.css stylesheet is what always loads for any one that isn’t a logged-in administrator e.g. regular site visitors, or members with other WordPress roles. And is updated when you click the green ‘Publish’ button.
So you should leave draft-styles.css alone. It doesn’t add any extra weight, and is useful for checking unpublished styles outside of MT (but logged in as an admin).
I hope that helps. Please let me know if you have any further questions.