I do prewash my fabrics to remove the chemicals. My hands get tiny itchy bumps if I don't prewash. Also I prefer to pre shrink my fabrics so that my finished project won't shrink so much. I also preshrink my batting if I am using a cotton batt.
If I am going to start the project right away I do HEAVILY starch my fabrics and press my fabrics. I think I get much cleaner cuts and the fabric is easier to work with when it has some body to it.
If they are fabrics for the stash I wait to wash them when I am ready to use them.