Pre-washing is purely a personal preference. However, having said that, I have always pre-washed; at first, because I was taught to do it when I learned dressmaking 40 years ago. Then, when I started working at the LQS, my boss told me about all the chemicals used to treat fabric so that insects and other vermin would not be attracted to it during shipping to the US. You wouldn't believe how sticky and nasty my hands get after a day of cutting fabric at the shop! Anyway, now I pre-wash everything just to get those chemicals out of the fabric before I start working with it. It's one of the reasons I don't normally use precuts.