As for the top that you completed. Wait until the quilt is finished and wash the enitre quilt with Synthropol. It will suspend any of the excess dye so it does not discolor any surrounding fabrics. You can use it for the backing fabric prior to use, or for this instance ( the backing fabric prior to be cut or sewn ) use retayne , but if you do the quilt must be washed in cold water.
I reccomend you wash the backing in sythropol as well prior to cutting or sewing, since you will not have any control over how the quilt will be washed in the future.
Yes I wash/soak precuts to remove any excess dye before construction. Precuts I wash/soak in the sink till the water runs clear. Hang to dry.