There are all sorts of people that will jump in here and tell you all kinds of reasons why you really, really, REALLY should wash your fabric or else!
If you don't want to spend a whole lot of time washing and ironing fabric, ignore them. It's a whole lot of work to wash and iron and starch 10 yards of fabric before you even get to cutting.
The main reason people give for washing seems to be dye bleeding. If it's not set in the original it won't be set in anything that picks it up and will wash out of that item too. Most fabrics don't pick fugitive dye up out of the wash anyway. Wash a red and white together. If the red bleeds and the white doesn't pick the dye up, it's not a problem.
One of the top quilters in the counry, Harriet Hargrave, does not prewash fabric. She and her daughter have written a series of books called "Quilter's Academy" and I highly recommend the first book for all quilters. Harriet has been quilting since the '70s at least so I'll bet she's had more than enough experience to be a true authority.