They don't have any offset in the package, but when you first take them out of the hot water you can give them a little twist. I did 6 arrows this weekend, and the first two had a bit too little twist, but the last four ended up with a nice and quite equal amount of twist, once I got used to how hard to twist them.
I shot a couple dozen shots, both with field points and Slick Tricks. I was a little rusty, but the groups were at least if not better accuracy than my non-shrinkwrap 4" Duravane. I was really pleased with how ick and queasy it was to fleetch them. The time savings was incredible! Once I had the shafts clean of the old fletching, and the water was hot, it took me about 2 minutes to fletch 6 arrows.
But I think Radar is right (as usual). IF one of the vanes gets damaged, I don't think you can remove it from the shrink wrap cleanly enough to add a vane using glue and a fletching jig. You'd have to re-do the whole thing.