Twist in the main (in varying degrees, depending on the wind strength and sailplan) is almost always needed to account for the vertical change in apparent wind.
Yes, some twist is good. But if you always center the traveler, for all points of sail including reaches in moderate or more air, you will be allowing too much twist for most cat sail cuts. With spinny boats the apparent wind is clocked around so high on the mainsail, the traveler is centered even when reaching. Other than watching the telltales, one way to know you have it set up wrong is excessive weather helm on an otherwise correct (no rudder rake/tow wierdness) boat. You can get any boat to have a heavy pull on the tiller, just head off the wind (no spinny) and keep the traveler centered and ease only the mainsheet. You get loads of weather helm.
Jimbo
