Your concern may cause a spike in the polygraph (may, I stress; nothing is assured with this augury).  I would check to see what the official status of your charge is.  In the state I live in, for example, if you're pardoned of a state offence, legally it never happened.  It states in our law that you can truthfully answer "no" in any court proceeding if asked "Have you ever committed a felony?" if you were convicted and then pardoned.

If it's "sealed", it may not have been dropped.  If you have a juvenile record, it may well be sealed and never used against you, but you would have still committed a misdemeanor.  Or you could have gone through a diversion program and had the charge dropped/dismissed/continued forever.  Ask a lawyer in your state, that you can be confident in whatever you say...

I believe you are morally obligated to answer the question truthfully.  Unless the charges were dropped, you do have a record, even if the details are sealed.  You're much better off being up front about it than having them find out about it 6 months down the road when you're already in your new job and life.  That way, you won't have to worry about it all the time.   

I would investigate very carefully whether or not the misdemeanor is really sealed.  Even if the actual trial/plea agreement/whatever is sealed there is probably a record of the arrest and booking in the NCIC.