To clarify, I'm hoping people understand that starting in the middle when machine quilting means starting at the middle "of an edge" and working your way to the other edge. If you start all your lines in the smack-dab center of the quilt, you will have a zillion thread ends to hand-bury later on.
When hand quilting, you start in the center of the quilt and work out to the edges.
When machine quilting, you start at an edge and work your way to another edge, so starts and stops are buried in the binding. It's best for the first line to cross the center going one direction (say, top to bottom) and the second line to cross the center going the other way (say, side to side). You then work your way out from those lines.
In both cases, working from the middle stabilizes the quilt and moves any excess fabric to the edges. (Working from the edges towards the center can trap excess fabric in the middle, where it has nowhere to go).