I hold GE. I have a felony arrest record.
All I needed was to provide paperwork that showed that all the charges form my arrest were dropped without a finding of guilt. My interviewer could see everything I was charged with and that all the charges were dropped except one charge that was dropped at the preliminary hearing. (Apparently he didn't have access to records from that type of court in PA.) I had to go to the district court to get paperwork that showed the charge was dropped at the prelim - I received approval within a day of faxing that over to CBP. Without that paperwork I would not have been approved.
My advice: don't lie. If you were arrested, assume the interviewer knows. Even if you disclose something that isn't known by CBP, it is NOT a bar to approval as long as there was no finding of guilt. Since your charges were dropped and expunged, worst case scenario is that you have to contact the court to get paperwork to prove that to CBP.