If I buy yardage, I do prewash. If it's fat quarters, which I don't buy often, I don't. I am a little sensitive to the formaldehyde (that is probably not spelled right :oops: ) that a lot of fabric is treated with, so I wash it. Otherwise, I cough and sneeze like crazy.
Sometimes the fabric looks just like it did when I bought it after it's washed, and sometimes not. If it looks "tired" after I wash it, I'll iron it with starch or sizing.