I don't use soap, but not because I cannot get the soap taste off - boiling an inch of water in the pan will do the job. As will heating the pan hot enough to burn the soap off, red hot being traditional, followed by re-seasoning. But there is a better way:
In the 1960s, I worked for a summer at a McDonalds making the shakes, and I watched the short-order cook (a 45 yo Texan). He just heated the grill sheet up, dumped some water on the surface, and scraped and scrubbed the boiling mess with a heavy steel scraper and an old rag. This was fast, and completely effective. He had to do this periodically, or the accumulated grunge would affect the taste of the hamburgers.
I still use the Texan's method, followed by smearing some grease (from whatever was last cooked) all over the now too-clean (rust-bait) surfaces.
Joe Gwinn