On the rare occasion that I use my cast iron on an open fire, I clean the outside with soap and water. If it is a real sooty mess, I pick up a handful of sand and scrub the outside with that. After using the sand and water, I switch to soap and water to clean off the dirt. Nothing like a hand powered sand blaster to get rid of greasy soot.
I also use soap on the inside of my cast iron pans and ovens. They are very old and well seasoned and I have never ruined the seasoning of the pans. I do not soak the pans in soapy water, I just add soap and water, scrub, and rinse.