You hear this sometimes from cast iron owners that think using some soap will "ruin" the seasoning. It's a myth, you can absolutely use soap. My preferred method is chainmail + coarse salt + small drop of dawn.
Yes, I use a little bit of Dawn when the pan is really greasy or crusty. Hot water in the pan, a little bit of Dawn, let it sit for a few minutes, scrub. Dawn is not agressive enough to remove the seasoning, it will just emulsify the liquid grease/oil in the pan.
Do not put them in the dishwasher though, or you'll have to re-season them.
Dawn kinda smells tho, especially when the pan is heated again for the first time. Whatever it pyrolyzes to, I'm not sure I want to smell or eat it. The store brand dishwasher detergent seems to not smell as much but if there's no debris from the food I avoid soap or use it very sparingly.
Good tip with the coarse salt, I'll have to try that sometime.
Do not put them in the dishwasher though, or you'll have to re-season them.