Bottom line: There is no evidence of whether or not continued use of IUDs in post-menopausal women poses any risks.
Details: DynaMed Plus entry on IUDs references these guidelines: Obstet Gynecol. 2011 Jul;118(1):184-96. ACOG Practice Bulletin No. 121: Long-acting reversible contraception: Implants and intrauterine devices.
Documents that there is no record of clinical trials that evaluate the risks from prolonged IUD use in asymptomatic menopausal women and reports that many post-menopausal women tolerate IUDs.