I've used cast iron all my life. I don't even normally use soap on it. Maybe plain hot water with a scubber if something is stuck to it, or you cooked tomatoes and stripped the finish, or it got rusty or something. 80% of the time I just wipe it out with a paper towel.
And seasoning cast iron is dead easy:
Get it hot,
Put some kind of grease on it.
Done.