Basically, the traveller sets the coarse angle of attack for the entire sail. I use a tell-tale pretty much in the middle of the sail to set the traveller.

The mainsheet adjusts the amount of twist (the variation in angle of attack from the bottom of the sail to the top) in the sail. I use two tell-tales up high (2nd and 3rd panels down, top one is 2/3 aft, bottom is centered) to set twist.

Leech tell-tales can fine tune the twist and keep you from being oversheeted, especially in light to moderate air. They are also up high on the sail, since that's where the largest variation in twist occurs.