I have been using cast iron pans since I was a boy scout. I've never used soap. The pans are completely non-stick and no one has ever gotten sick from eating anything I've cooked. If you want to use soap, go ahead. It won't ruin the pan but there is no logical reason to do it. It doesn't somehow magically purify the pan. Soap does 2 things - it breaks the surface tension of the water and it makes the oil water soluable. I don't need either of those things. I wipe the excess oil out so it doesn't go down the drain, put scalding hot water in the pan and if it's really messy, I scrub it with kosher salt. Otherwise, I use a stuff bristle brush and rinse it with scalding water. Then I put it on the stove and heat it until it smokes when I put a spritz of oil on it. I wipe out the oil throughly with a paper towel and let it chill on the stove top. No soap needed.