I tend to prewash most fabric as I get eczema from chemicals in some fabrics.
I learned the hard and expensive way to starch when I am planning a quilt that will have bias edges. I finished the quilt I was making, but am not at all happy with the results. It was meant to be a gift, but I could not give it to the recipient.
No, I am not a perfectionist and do not worry about cut off points etc. The quilt I was making had 8.5 inch blocks, that grew after being squared up. When I squared them up the second time I lost quite of bit of the design that created a beautiful secondary design.
I have learned that if I need HST in a design I used methods to make them that do not result in bias edges on the outside of the blocks.
I use homemade starch using cornstarch from a recipe I found online. I make it up as needed for each project.