The leech line, as Bob says is to control flutter. It should be left completely eased off until needed and then tensioned just enough to remove the flutter. This applies to head sails as well as the main.
As a practical matter, I have my mainsail leech line adjustment at the luff of the main about a foot above the goose-neck. On a run or reach it is impossible the adjust the leech line if it is located just above the clew. Moving it to the front of the sail allows for adjusting at any time.