I have been quilting almost 40 years and have never considered prewashing a batting! I purchase the batting I want for the project. There are so many wonderful battings on the market now to choose from and many do not shrink or if they do it is very minimal. Dream Wool and Hobbs wool are my favorite s, I have used the silk which was wonderful to work with. The package yours came in should have measurements so you can see if you stretched it out. I've never found a packaged batting that didn't have wavy edges. Even that dense, flat warm & natural has some edge stretching out of the packaging. I really do not understand all the extra work some choose to take on, today's battings are wonderful right out of the package.