I don't prewash fabrics, so for general cutting I don't starch; there is enough sizing in the fabric off the bolt to keep my cutting and piecing accurate.
Backing fabric and fabric to be cut into bias strips I *always* starch heavily with the method I think I described in the other thread (1:1 solution of Sta-Flo and water). I do not prewash before painting the starch on.
The only fabric I will faithfully prewash and dry (twice) is flannel. Unless it's a rag quilt (which I haven't made yet, but which everyone says not to prewash the flannel for), I then use heavy starch on the flannel fabric before cutting it. Heavy starching keeps the flannel from stretching while I cut and piece, making accuracy while piecing very easy.
For backing fabric, I find the heavy starching prevents puckers on the back when I machine quilt. (Would not do this if hand quilting!)
For bias cuts, heavy starch stabilizes the fabric so the strips are even and do not get stretched out from handling while sewing. It also makes any ironed-in creases very sharp.
I usually don't use spray starch because I am so clumsy with it -- always making a mess with over-spray, and seemingly always using too hot an iron on it while damp and scorching it. However, if I had made up a quilt sandwich and wanted to machine quilt, I wouldn't hesitate to spread out a sheet on the floor and spray starch both the backing and the top to prevent puckers while machine quilting.