Need urgent help!

I ever got arrected around two years ago, because of drunk driving. But the case was dismissed. Now I'm applying for green card. Do I need to tell that I have been arrested before when I fill in the form and in the interview?
I asked lawyers, some said I don't need to do that since the case was dismissed. And some said I still need to tell about the arrestment, since the finger print is still there and the Federal gov will see that.

Really not sure what should I do.