Is it really necessary to avoid using soap on a cast iron skillet?
Here's the deal ... if your pan is seasoned correctly using an oil with a high iodine value, then no, soap won't harm it.

If it's not seasoned correctly, then it could need a strip &amp; reseason.  If you're using a low iodine oil (the surface will be slightly tacky when the pan is cool), then it also might ablate the surface somewhat, and repeated cleanings could damage it.

If you think it needs a deeper clean, one approach is to always add a new (thin) layer of oil after cleaning the pan (and then heating it 'til it smokes) ... this means that you'll have a relatively thick layer of seasoning and if you lose an individual layer, it's not a big deal.  (and you'll add it back right after the cleaning).