In general, you want to start quilting in the middle and work your way out so that any excess fabric is pushed to the edges. If you were to quilt all the sides and then start quilting towards the middle, you could end up with a large "bubble" in the middle that wouldn't "quilt out".
Having said that, it depends somewhat on how you are planning to do your SID.
If you are planning to quilt around each square individually, then you do want to start in the middle and work your way out in a circular fashion.
However, if you are planning to stitch entire lines at a time, I would start *in the middle of an edge* and quilt from one end to the other -- e.g., start at the middle of the top and stitch continuously from the top to the bottom. My next line would be from the middle of the side across the entire quilt to the other side. This would still be working from the middle out, but an entire line at a time.