I would probably wait to spray starch until after you have completed a block. At that point you can use spray starch to "block" the block to correct size.
Starching before cutting, just as with washing before cutting, is a personal preference. I don't starch fabrics before cutting, but then I also don't prewash. Starching, for me, is not necessary for fabric right off the bolt because that fabric already contains some sizing from the manufacturer. If I need to wash a fabric, however, washing removes the manufacturer's sizing and can leave the fabric limp. That kind of fabric I will starch before cutting, to restore some stability and stiffness to the fabric.
Did you wash this fabric before cutting out the pieces? In that case, you do have the option of spray starching before piecing, as you can be pretty sure the cut pieces won't shrink or bleed. If the fabric was not pre-washed, you run the risk of the pieces shrinking or distorting if you spray starch. Besides, unwashed fabric usually has enough sizing in it so that starch is not needed.
In a situation where I need extreme exactness in piecing, I would heavily starch unwashed fabric before cutting to ensure extremely precise cutting and piecing. A few quilters do this routinely. I just skip any step that I think is not absolutely necessary, as it takes me long enough to complete a quilt already!