I (soak in hot water and then) wash everything that is washable before cutting it.
I only started the soaking in hot water before washing a few years ago. I learned that a quick "swish" in water did not remove much excess dye. Learned that when pressing some dark blue and dark red - washed - fabric that I had sprayed sizing on. I was not happy with the bleeds on to the pale gray fabric that I was using. They were Jinny Beyer fabrics by RJR.
If a large piece of fabric has something in/on it that I want removed or modified, then a small piece of matching fabric would have the same characteristics.