It really depends how the question is worded when asked. Most say something to the effect of "have you ever been convicted of anything more than a traffic infraction" or "have you ever been convicted of a misdemeanor or felony." If this is the case, a sealed or expunged record will allow you to say "no."
Where it gets to be a bit unfair, in my opinion, is when asked "have you ever been arrested- state date, charge, and disposition." You could have been wrongfully arrested while walking back to your office from Starbucks as you fit the description of the guy who just robbed the bank across the street, but you'd have to disclose that, as, technically, you were arrested.
As far as I know, the FBI background check (which I'm told is the same information that a 121 carrier will pull from the Fed) will show convictions, but not arrests.