Hi jmore,
I'm watching, "Small Machine, Big Quilts, Better Results," with Ann Petersen. I'm not finished watching it yet, but I think that I've gotten through the chapter explaining why she starts from the outside. Her idea is that she doesn't want to try to squeeze all that bulk of the quilt into the harp of the machine, like you would if you started in the center. She stablizes the whole thing first with stitch in the ditch, so each section is going to stay put, so no worries. This also leaves a straighter, outside edge because if started in the center, there might be some fabric pushed out to the sides and borders would be misshapen with all that quilting and shoving things around.
I'm also taking another class with Marti Michell that explains techniques on how to divide the quilt up and quilt as you go, so you don't have to deal with all that bulk. With her method, you can also start your quilting anywhere you want.
~ C