According to the following article, approximately 96% of physicians accept Medicare Assignment.
Paying a Visit to the Doctor: Current Financial Protections for Medicare Patients When Receiving Physician Services | The Henry J. Kaiser Family Foundation
So I'd say the odds of incurring Part B excess charges are small. A person can also look up their doctors (or the agent can do it for them) on the Medicare website and see if the doctor accepts assignment or not.
You're my hero.
I pitched N today, got into this discussion and he wanted "proof" it less than 5% of docs nationwide and finding something like this was on my to-do list.