You have received a lot of great advice on this thread! My personal recommendation is not to do SITD but instead just use the edge of your walking foot as a guide to sew down both sides of the seam line in a light colored thread. Grid quilting like this looks great and is much easier to do than SITD. And no marking needed.
Then you could do a line or two down the middle of each block depending on how much quilting your batting calls for. Or do a diagonal line using painters tape as your guide.