Another way to set your jib twist is to sail to an upwind setting and fall off the wind. Look for both upper and lower tales on the jib to break (stall) at about the same time. This is probably more important that comparing the main/jib twist. This way you know your jib luff is pretty parallel to the breeze and the top and bottom trim match and the sail is providing an equal amount of driving force all the way up. However, if you're bobbing and weaving in the waves, a little bit of twist in the top will probably keep more air attached (some part of the sail will be in good trim as the boat 'tomahawks' through the air) and drive a little more consistency...so you want a little twist in rough conditions and none in smooth stuff.

I think most of that came out of Rick White's book.