"It often makes you 'joint and severally liable' for any problem, " says Carlton Smith of Cardozo Law School in New York.

WSJ: Tax Report: A New Push to Protect 'Innocent Spouses'

But if the couple files a joint income tax return, then each of them will be jointly and severally liable for the income tax reported, or properly reportable, on the joint return.

FORBES: IRS Expands Availability of Innocent Spouse Relief