Technically you don't HAVE to cover anything. If you detail the engine like you are putting out a fire and expect a clean engine to come out of a can, then you probably do, but if you a good job by hand, then you don't need to cover things. Just avoid spraying degreaser on ANYTHING electrical (alternator, fuse boxes, battery, any exposed electrical connections, etc) and you should be good. Don't spray any exposed filters either. This has my take on engine detailing. You can get it quite clean without covering things if you are being careful and professional about it.