Something not well known in dogs is gland problems, and one of the most common glands to have a problem is the one groomers and vets usually drain when your dog visits them. Some say it's beneficial to drain it, some say it isn't. Do you think it's beneficial for the dogs, or should they stay unsquished? Talk to your vet/ groomer about it.
My dog had this issue. I only expressed the gland when there was swelling and inflammation. This was something that didn't happen often. Not sure why it even occurred. My vet showed me how to do it. I only did it when it didn't drain on it's own.