Obviously, you will have to await diagnostic testing testing before proceeding with any treatment plan. But in answer to your question, in my own mind, the decision whether or not to treat depends on a number of factors: age of the dog, apparent discomfort, severity of symptoms (both external and internal), etc. Cushing's is typically a slowly developing syndrome, and the decision to actively treat may come sooner or later in the process. The primary goals of treament are to control the troublesome observable symptoms (things like excessive thirst, urination, hunger, panting, muscle wasting), and also to halt/delay silent internal damage. Untreated Cushing's can make dogs more vulnerable to things like high blood pressure, high cholesterol, protein-losing kidney disease, blindness (as a result of high blood pressure), pancreatitis, ligament damage and slow healing, chronic infections and skin issues, GI disturbance, etc. Not every dog will suffer from all or even many of these issues, but the disease can make them happen.
Since the damage usually occurs over time, my own opinion is that I don't worry so much about the "silent" damage in a dog who is already elderly. My biggest concern for a senior dog is immediate quality of life. So if the overt symptoms are making the senior dog uncomfortable, I would treat. Otherwise, I might not.
However, for a younger dog, I would be equally as concerned about the silent internal damage. I would want to give my dog as many quality years of life that I could, and that would mean putting a stop to the development of the internal systemic damage as well as relieving the uncomfortable outward problems.