Your quilt is beautiful! You did a great job of the piecing, and getting all the sashings lined up.
If I was you, I would quilt beside the ditch rather than in it. Since you have a walking foot, quilt lining the side of the walking foot up with the seam. Your quilting line will then be parallel to, but about 1/2" away from, the sashings.
Since your blocks are quite large, you should probably have some stitching through the middle of them. Even if it's not required for your batting, it will make the quilt last longer. I would do diagonal lines like someone else suggested. I've made a little diagram of what the quilting would look like:
[ATTACH=CONFIG]566544[/ATTACH]
As an aside, like Watson said, spray basting won't leave a sticky mess... I spray baste because it makes it so much neater to redo things. The spray is only very slightly tacky, but adheres well to fabric. I wish someone had told me about spray basting when I was just starting to quilt, as it makes basting SO much easier, and means you don't have to avoid pins when quilting.
Good luck - I'm sure it will turn out great, whatever you decide to do!