If I have been convicted of a crime, but my record has been expunged, do I need to write that on my application or tell a USCIS officer?

Regardless of whether your record was expunged or sealed, you must disclose all criminal arrests and convictions to U.S CIS officers and on any Immigration application you submit. In order to properly disclose your record, you must provide certified copies of the arrest report as well as the disposition of the case.