Actually, you do want to wash it in hot water and dry it as the shrinking tightens up the weave to make for a stronger patch and removal of the sizing allows the lubricant to be better absorbed throughout the fabric. Wash the whole sheet of fabric to minimize the fraying at the cut edges. You could of course hem or simply stitch the cut edge to minimize fraying, but the width of the hem is just about the amount of material that will be lost due to fraying. Review the tips that
@Coinneach posted above with his thread of patch material.
There are more benefits to be gained from washing the patch material than not.