Large squaretop sails like a bit of twist along the leech. Pulling the leech tight on these sails with slow the boat down. The explanation given for this is often the unacceptable increase in drag because of having a large tip vortex.

On the French Sailing DVD Glenn Ashby says in an interview that his top is often around 300 mm twisting off, as taken from what would be a perfectly straight leech.

But you can feel it too. If the leech is to tight up top then the boats hangs up, it feels powerful but not fast. You heel easily but don't accellerate quickly in the gusts You don't get that nice "streaming" feeling that you would get with the right amount of twist.

Try it a few times, you will learn to feel it.

Wouter