At Muddy-Mutts I go by the saying "Humanity before vanity". I groom humanely and will only attempt to de-matt a dogs coat if I know the dog will not suffer any pain by using specialist equipment in my parlour. This being my first option before shaving that part of the dogs coat.
It is the owners duty of care towards their dog to maintain the coat between professional grooms. If this has failed to be done, severely matted area will be shaved off to prevent unnecessary suffering. To attempt to de-matt would cause the dog tremendous stress and discomfort and some dogs can be traumatised by unnecessary hours of de-matting putting them off grooming for life. Shaving the coat would be quicker (saving the length of time the dog was on the grooming table for) and kinder to the dog.
When a dog is severely matted my only option would be to shave the whole coat using clippers.
I would always talk through the options with my customers and inform them if this had to be done before grooming their dog.