The only reason I have ever heard of using soap on snakes for is to break the water tension for treating mites. Even then its only a few drops of soap.

For anything else, some diluted chlorhexidine solution works well to clean them up, but to be honest I dont do it unless they clearly have poo or urates on them. I just think it's best to leave them be. There are things such as good bacteria and bad bacteria. I don't wash them with anything besides water, because I wouldn't want to harm anything I didn't mean to.