My LQS owner told me that quality quilting fabric doesn't need to be prewashed unless you are super sensitive to dyes, etc. She said that today's manufacturers are very particular about making sure their fabrics won't bleed onto other fabrics. Batiks are another story, however. So, I don't always prewash everything. If I buy anything at a yard sale or someone gives me fabric, I wash that just because I don't know how old the fabric is.
So, I wouldn't prewash the items you're using.