I agree that it's rarely ever necessary to bathe a snake. If I felt I had to do so though, for whatever reason, I'd choose Dawn...plain ol' original Dawn (none of the fancy scented stuff). It's the go-to cleanser for every wildlife rescue I've ever heard of. It's also recommended for treating mites. (It won't kill off an infestation, but if used properly it will drown the suckers that are currently on a snake) Even so, I'd use it very sparingly...just a few scant drops in a sink of water.
Only reasons I can think of to use it:
1. Drown mites crawling on a snake
2. Frozen/thawed rat exploded all over snake during constriction
3. Snake crawled through runny poo and had a LOT of it dry on its body