For what purpose? The only address you need to put on your tax return is where you want to get tax-related mail. It does not have to be the place you live. As a US citizen, your wife is required to report and pay US tax on all her worldwide income, even if she is living outside the US. (She can claim a deduction or credit for taxes paid overseas on the same income.)
Her residency would only be important if she qualifies for the foreign earned income exclusion, which requires that she live overseas at least 330 days of the year.
[Edited to add]
Her foreign residency could also be important for state income taxes, as mentioned below, but only if she has changed her domicile, which is her real permanent home and the place she would return to if the foreign assignment ended.
