The reason people used to not use soap on cast iron skillets is because soap used to have lye in it which would strip the seasoning from the skillet, most dish soap today does not contain lye and should be used to help properly clean the cast iron.
Also whenever I am helping someone with a new cast iron I always tell them not to worry too much about ruining the seasoning, because I have tried to strip a skillet of its seasoning completely by hand and it was so hard to get it all off
Which MMO?