Mirozen is correct, the never use soap on cast iron is culinary folklore and bunk.
The tough polymerized coating on cast iron pans resists normal cleaning agents like a dish detergents or soap. (Who uses soap these days?) Just don't use an abrasive, except salt, which can scratch the surface away.
To best maintain your pan, rather than using olive oil, heat on high and wipe on a thin film of high temperature tolerant oil (canola, corn, flax, etc.) after cleaning and drying your pan.