Having a shower won't prevent a WL from getting a throat infection if the customer has some STI on his penis. The girls use the shower to inspect the guy's penis to see if it looks clean but we all know that some STIs don't have obvious symptoms. In this case, the girl has every right to refuse to perform a sexual act if she feels there is a possibility she might get infected by a customer.
I remember one WL telling me how she had worked at one shop before moving to another because the boss didn't take the health safety issue seriously. One of his regular customers had some discharge from his penis and wanted a BBBJ from this WL. She refused to do it and he complained to the boss who tried to convince this WL to see him. She refused the customer and told the boss off. As a consequence, she didn't return to that shop to work.