I do expect a little extra to make up for cuts that aren't straight. I, too, have noticed that most of the LQS cut the exact amount with a rotary cutter, so the cut should be adequate and straight unless it's crooked on the bolt. And some of the cutters at JoAnns try to cut the exact length, but sometimes let their fingers slip so that it's a little short. I've noticed in the shops that do cut generously, you usually get about an inch over, regardless of the length you want.