Prism99

Yes, you would use spray starch for the quilt top.

For the backing, however, what I do is mix a 1:1 solution of Sta-Flo liquid laundry starch and water, "paint" this onto the backing fabric with a cheap, large painter's brush, allow a couple of minutes for the fabric to absorb as much starch as possible, toss the yardage in the dryer, then iron with steam. As long as you iron somewhat with the grain (in other words, don't try to push the iron along the bias!), you will end up with a very flat, very stable backing.

For the quilt top, you will not add any more distortion by "pressing" with spray starch. When I looked at your previous photo of the top, I think some of the distortions were from cutting and sewing issues; not all of it was due to your ironing methods. You may be surprised by how much of the distortion becomes unnoticeable after quilting and first wash.

With such a small quilt, and using W&N, you really do not need to add horizontal stitching. However, lines 10" apart are not enough for a crib quilt which will get washed frequently. 4" or 5" apart would be as far as I would go. Closer than that would be fine too; however, a lot of really close lines using regular 50wt/3-ply thread can add a surprising amount of stiffness. If quilting lots of close lines, I would use a polyester thread or finer weight cotton thread.

Are you planning on using a walking foot? I highly recommend that.

If you Google "wavy machine quilting walking foot" and then click on "images", you will find lots of examples of wavy line quilting for inspiration.