Never really found it necessary unless they get crusties on their cloaca or have trouble shedding. Other than that, why? I mean, it just seem like a good way to traumatize and stress your snake, especially if the temperature isn't exactly the same as the snake at the time.

Forget it. And forget the soap. Snakes don't need it and it could be potentially harmful if it gets in their nose or mouth. If they get poop on them or something, wipe it off with a damp rag and forget it. Their skin is non-porous, water-tight, they don't sweat, I mean, what's the point?

If your water dish is the right size, your snake will bathe when it wants to.