Any time we have to make a cut and there's a seam to follow we'll usually use a box knife to score the metal using the seam as a guide. It's hard on the blades and thirty year sheeting is thick and tough. But after a couple of passes with the box knife we can fold the metal back and forth and it'll break clean as a whistle.
Keep in mind I carry all kinds of metal snips, a plasma torch, a Bosch nibbler, and a Milwaukee hand shear. But on long lengthwise cuts where I have a seam as a guide I prefer a box knife.
It just looks hard. /forums/images/graemlins/grin.gif