I would test a square. Measure first (to check for shrinkage), soak in hot water for a couple of minutes, then place half on paper towels and half on a piece of the white fabric you will be using in your quilt. See if any dye transfers. Some whites will repel certain dyes so the most important to check is the white fabric. Even if you get a little bleeding on the paper towel, if the white fabric doesn’t pick up the dye (or if the dye washes out of the white fabric completely), you are safe not to prewash. Measure the shipbrokers square after it dries to see if it shrank significantly.