The reason they want you to go from the center out is because if the layers shift a little, it moves the extra out to the edge so you won't get a pucker. It doesn't really matter if you go horizontal or vertical first but I tend to do the longest one first so that any shifting is out to the edge. With any lines that cross over another stitched line, the problem area will be where those 2 lines meet. I am careful to keep the section secured well with my hands (kind of pulling the sandwich outward) as it get close to the needle.
If you decide to stitch across the whole quilt rather than center out, be especially careful where the lines cross. Good luck and with the sandwich spray basted, it should go well!