You have been in business for a long time and probably have clients from other States. Do you file and pay State taxes to each of those States where your clients' offices are located in addition to the State where your business is located? There are only 10 States that do not have State Taxes.
THIS depends on how your business is setup. I am an LLC because my billing company is a division of another business. I do NOT have to file by state for my out of state billing clients.
There are too many exceptions to this rule.. for example.. IN NJ.. You can be based in another state and if you do business in the state of NJ you must have your business registered and be registered as a third party billing company in NJ.
In NY.. if your business is located in any state but you do business in NY, you won't want to charge % of collections since in NY it's illegal for physicians to enter into a fee-splitting contractual relationship.
My answer is to consult with an attorney in addition to your accountant. There are so many laws/regs dependent on state domicile and doing business in that state that it's best to consult with attorney in addition to an accountant.