I also do not prewash. If I suspect a fabric might bleed, I test a small piece in a glass of water and rub the wet fabric against a piece of white fabric. If there is more than minimal color bleeding or color transfer, I will wash that fabric separately in Retayne.
Fabric comes off the bolt with sizing in it, which helps the accuracy of my cuts. If I prewash a fabric, I always starch it before cutting to restore stability to the fabric.