I am not a real expert at all of this, but starting in the middle is the IDEAL ... I have done quilts from one corner to the next, working on a diagonal.
You have sashing between your blocks, and you want to stitch in the ditch along the sashing, correct?
If that is the case, by all means, go as close to the middle sashing as you can ... the main reason for starting quilting in the middle is because on home machines the throat is so small, that the easiest way to move the bulk it to start in the middle, working toward the right (your roll will get smaller, while what is laying off to the left of your machine will get larger), then, roll the quilt back up, turn it around, and starting on the other side of your sashing stitch in the ditch on either side of the sashing. To get the side to side sashings stitched, roll the quilt narrow ends in toward the middle sashing, once you have that rolling done, you will be able to stitch at any interval from the middle to the outer edge (in both directions, just one direction at a time)
Now ... another reason for working from the middle outward is in case there are some bubbles that have to be worked out, but I wouldn't worry about that in your case.