I am not a prewasher unless I suspect bleeding. I have never prewashed a precut. I was given three charm packs that had been prewashed. A mess, none were the same size and I ended up trimming them down to 4.5 to make them the same usable size. Some shrunk less than 1/16th inch, while others nearly 1/2 inch and were wonky. The pinked edges were a no longer sharp points and I cut them all off. Poor quality fabric? Poor cutting of the charm pack? Maybe, but they were glorious colors and the fabric had a good "feel" so fussed with them and made a donation quilt with them.
I have never tried this but I recall a suggestion to soak precuts in moderately hot water then put them in a "salad spinner" to get the water out. The idea was that this would be easier on the fabric and not damage the edges. What? Who still has a "salad spinner"? I choose to just use the jelly roll or charms or layer cakes as is.