From the FAA:
FDC 4/4386 FDC SPECIAL NOTICE...
NATIONAL AIRSPACE SYSTEM INTERCEPT PROCEDURES. AVIATORS SHALL
REVIEW THE FEDERAL AVIATION ADMINISTRATION AERONAUTICAL INFORMATION
MANUAL (AIM) FOR INTERCEPTION PROCEDURES, CHAPTER 5, SECTION 6,
PARAGRAPH 5-6-2. ALL AIRCRAFT OPERATING IN UNITED STATES NATIONAL
AIRSPACE, IF CAPABLE, SHALL MAINTAIN A LISTENING WATCH ON VHF GUARD
121.5 OR UHF 243.0. IF AN AIRCRAFT IS INTERCEPTED BY U.S. MILITARY
AIRCRAFT AND FLARES ARE DISPENSED, THE FOLLOWING PROCEDURES ARE TO
BE FOLLOWED: FOLLOW THE INTERCEPT'S VISUAL SIGNALS, CONTACT AIR
TRAFFIC CONTROL IMMEDIATELY ON THE LOCAL FREQUENCY OR ON VHF GUARD
121.5 OR UHF GUARD 243.0, AND COMPLY WITH THE INSTRUCTIONS GIVEN BY
THE INTERCEPTING AIRCRAFT INCLUDING VISUAL SIGNALS IF UNABLE RADIO
CONTACT. BE ADVISED THAT NONCOMPLIANCE MAY RESULT IN THE USE OF
FORCE.
(My bolding). Bit subjective. I guess if you've got other things to do with the radio you don't have to monitor it. Might get shot down though!