This question comes up far too frequently in various forms on Avvo. It can be a question regarding an arrest or a conviction.
If the question does not exclude the arrest/conviction, then it must be disclosed. If there is any doubt that the arrest/conviction is excluded, one should err on the side of disclosure.
USCIS is likely to have a record that the arrest or conviction happened. That is one of the reasons why immigration applicants are fingerprinted, so that USCIS can run their fingerprints through the FBI databases to determine if the applicant has arrests or convictions.
You do not do yourself a favor by failing to disclose. You merely complicate your case, and risk being charged with lying on an immigration form, and the loss of the immigration benefit you are seeking in the first place. If the arrest or conviction renders you deportable, failing to disclose it is not going to save you.
That being said, if you do have an arrest or conviction, it is imperative that you have an immigration lawyer review the arrest and conviction records BEFORE you file anything with USCIS. Arrests and convictions that have minimal consequences in criminal court, can potentially render you an aggravated felon for immigration purposes and can and do result in deportations.