One of my cats (Pixel) LOVES to do this, and does it almost every time I shower. I always make sure to rinse out all the soap (and wipe with a washcloth usually) to make sure that there is no residue. When I clean the shower, I rinse great and keep the door shut all that day and rinse again the next day before I let her at it.
That said, I think it's fine. She licks the water out of the bottom of the tub and gets her feet, belly, and butt all wet. I think it's hilarious... but not harmful.