This is a hotly debated topic, and at some time, you've probably been told that your cast iron will be RUINED if you use any soap or soapy water to clean it. You can relax. A good scrub in soapy water isn't going to destroy your skillet. Don't let it soak in soapy water, and don't pile on the dish soap in the hopes of getting it sparkling clean, but it's fine to wash it as you would any other piece of cookware. Remember to re-season it as needed.