Don’t Mess With The IRS
While there are no set rules, there is one situation in which the assistance of a tax attorney or professional is necessary: any criminal tax matter. If you are being investigated by the Criminal Investigation Division of the IRS or are being interviewed by an IRS Special Agent, be sure to consult a lawyer experienced in criminal tax matters. Stop talking and get an attorney.
While other tax professionals now have a confidentiality privilege in various tax planning matters, only attorneys have the confidentiality privilege in criminal matters, and typically only attorneys are experienced to handle criminal tax matters. In most other matters, whether you need the assistance of a tax attorney or professional is really a matter of discretion. However, tax law and IRS procedure can be extremely complex and well beyond the ordinary knowledge of even experienced business people.
The IRS is a massive bureaucratic organization. Like any such bureaucracy, it can be incredibly frustrating to deal with. Finally, the IRS has extraordinary collection powers far beyond those of ordinary creditors. Given this combination, the benefits you might get from the assistance of a tax professional may very well offset the costs of professional fees.