It would be advisable NOT to disclose your arrest for petit theft which was expunged. If you were to disclose it, then it could be used to impeach your credibiility.In the event that the  petit theft arrest is discovered  by the opposing party, you would want to emphasize that there wasn't any conviction and that it was expunged.  If this arrest occurred long ago and you haven't had any criminal record since, emphasize that if the other side raises the issue.