The idea of working with precuts is that you are dealing with an exact measurement right out of the gate. Any time you wash the precut it WILL skew the size. 5" square may not be 5" any longer and I guarantee that it won't be square. Don't even want to think about what happens to a jelly roll strip. So that negates the point of working with precuts.
With the availability of color catchers, prewashing is no longer such a hot requirement imo.