Of all the concerns of cooking in cast iron, cleaning seems to be the most discussed of all followed by seasoning. Cleaning of any cooking or serving items, including cast iron, basically revolves around fastidiousness, hygiene, personal taste, and care for the cookware itself. Using Lye (soap), Detergent, Ammonia, Coarse Salt, Vinegar, Cornmeal, Boiling or just wiping the pots have all been advocated as cleaning techniques and it looks like everyone has there own technique that works for them. A review of these techniques seems to be in order so the reader can decide on which best suits them.
Cleaning in general involves removal of unwanted soil from the surface of the particular item in general. Cooking vessels generally involve soiling from both oily soil and baked on soil in the process of food preparation. Baked on soil usually follows some form of abrasive technique to remove while the former can be removed by other techniques involving a detergent or other surfactant to roll up the unwanted cooking residue. Removal of these soils is a consideration of both hygiene and care of the vessel itself.
Specifically addressing cast iron cookware, in particular the "Black Iron" variety it's important to review its surface. Almost all of this type of cookware manufactured today comes pre-seasoned in which some form of oil is applied to the vessels surface under pressure, then baked in under high heat. This baking allows the pores of the metal to expand, absorbing the oil, and upon cooling seal the surface against moisture so as to prevent rusting. What makes cast iron unique in this respect is that surface surface damage event, it can be renewed by re-seasoning unlike other cookware which must be discarded.
Addressing the aforementioned cleaning techniques, most manufacturers of decent cookware dissuade the use of either harsh or abrasive materials in the cleaning process. Lye (sodium hydroxide), Ammonia (Yuck), and Vinegar (acidic) unless properly diluted can affect the pans surface and complicate the cleaning process as does boiling due to having to refill the pan and bring to a boil although these may be effective for heavily burned on soil. Abrasive techniques such as coarse salt and cornmeal can also affect the surface, as does dishwashers. Merely wiping may be OK for dry foods but for oil based soils is not particularly good from a hygienic standpoint.
Regarding the use of soap or other surfactants on cast iron, a balance between cleaning, hygiene and personal taste must be made. Cast Iron Cookware has a tendency to retain a flavor of previously cooked food which some delight in so they clean to favor this characteristic. Others may prefer a deer cleaning to remove the flavor bearing oils required by this tension so this remains a personal choice. Cast Iron is porous, as is a lot of cookware, and surfactant absorption is a factor of contact, concentration, and time. Here we need a balance between cleaning for hygienic reasons and yet retaining the cookware's inherent properties.
A reasonable balance could be stuck by employing the "Neat Method" (ref.1) of cleaning which removals baked on soil using hot water with a mildly abrasive cleaning device followed a mild detergent applied to the same cleaning device is employed. This is followed by a thorough rinse in hot water and drying with papers towels. In addition, I also employ what I call intermediateary seasoning by warming the pan over a medium heat, spraying with cooking oil, cooling, and wiping off the excess. This has worked well over the years with frequently used pots and pans. For consideration on detergents and cast iron cookware see (ref. 2).
In conclusion, while there is no definitive cleaning technique for cleaning a cast iron pot other than the manufacturer's instruction, or personal preference, these are some considerations which should be kept in mind if you're considering acquiring one of these great cooking vessels.
1) Liquid Detergents, Kuo-Yann Lai, P214 -155 Washing methods
2) Dawn Detergent, http://dawn-dish.com/en_US/dishwashing-tips/the-dish.do