I never used to prewash because I was too lazy and often come home with fabric i want to use right now. One of the owners of a LQS told me she has her employees wear gloves when they unpack fabric and encourages them to wear glove whenever possible because of the chemicals the fabric is treated with. She said it is also treated with insecticides. She said the shipping containers it comes in from overseas were loaded with dead bugs. I have been prewashing ever since we had this conversation. GROSS!