Dawn dish soap is fine to use in the water, just a drop or two, but I'd add it once the snake is already in the bath since they usually like to drink for the first bit of it. I have had to bathe my snakes, especially after a messy feeding two days later and they have dried rat blood on them...it's gross and nothing a bath can't fix. Nothing wrong with baths, it's not necessary but certainly won't harm them. I try to as little as possible for my paranoid/shy snakes, but my other ones wouldn't even care even if I did it every day (which I don't...just sayin')

My 4 y/o son handles my snakes, so baths after a messy poo and messy feeding is best for me. He washes his hands after handling but still, I know I wouldn't want to smell like snake poo or dead rat...even if I was a snake lol.