Absolutely essential.
There's a lot of chemicals (shampoos) and sharp objects involved. And there are other animals around. Shampoos might irritate the skin of a dog with dermatological issues. If the pet has a transmittable illness, specific procedures are typically used to segregate the pet from others to decrease the chance of transmitting the disease to other pets.
It's not just for the safety of your pet, but for the safety of all the pets in the facility. How would you like it if your healthy pet was attacked, and on top of that the pet had a transmittable illness? Making sure that your groomer is informed is best for all parties involved.
