The optimum (in my opinion) is that leech telltales stream aft, but break forward now and again. That way your sail is generating maximum power and minimum drag.

If the leech telltales stream aft all the time, you are probably not taking max power out of your sail but still have the same amount of drag.

If the leech telltales break forward all the time, airflow separates from the leeward side too early and you are loosing power and pointing ability at the same time as you are generating lots of drag.