Kristie -
Although you can get a lot expunged (especially prior to age eighteen), it is always there, whether it be in local police records or court documents. That being said, I think most airlines just complete a National Agency Check (NAC), which is basically a "run" through our National system (NCIC). MOST of the time, NCIC only includes major crimes (terrorist-affiliations, warrants, etc.). This is not to say you should lie if asked, though.
I should also note that "owning up" to past indiscretions says a lot about character, as many try to displace accountability, or worst of all, cover up incidents.
Keep in mind that my experience is with the law enforcement hiring/security clearance process, which, for the most part, is much more extensive and probative than the civilian aviation process.
J.