I learned at our Guild that using Sizing helps to stabilize the fabric before cutting, and after completing a block, or a big section you have completed.
That was 5 years ago that the spray sizing was talked about at our Guild. At that time, they were telling us to not use spray starch, as it attracts insects and moths, esp. So, the sizing didn't, they told us at that time. It could have all changed by now, for all I know.
That is what I like about the quilting board; you get all sorts of opinions and ways of doing things. We are always learning!
Mariah