I gotta plead on ignorance on this one; it sounds perfectly PC to me.
Some Internal Revenue Service employees continue to use the term “tax protester” to refer to taxpayers despite a 1998 law prohibiting the use of the designation, according to a new report from the Treasury Inspector General for Tax Administration. TIGTA found that out of approximately 257 million records and cases, there were 54 instances in which 45 employees referred to taxpayers as “Tax Protester,” “Constitutionally Challenged,” or other similar designations in case narratives on the computer systems analyzed for the report. Section 3707 of the IRS Restructuring and Reform Act of 1998 prohibits the IRS from labeling taxpayers as Illegal Tax Protesters or any similar designations. However, a few IRS employees continue to refer to taxpayers by these designations in case narratives, according to the report.
Should we go with "mentally challenged" then? Or would we be offending people with OCD, clinical depression, etc? Seems like a gray area.