Hi immabee,

I am applying from EB3 so hoping to be current next visa bulletin. For your case you should be fine since serving alcohol to minor is not a big case for USCIS specially since it doesnt falls into Crime of Moral Turpitude and yes you are correct we need to disclose everything even the case has been expunged.
For me since it happened while working in gas station when I was in F1 status it will be little hard if they question about my working status at that time.
I have consultation set up with an immigration attorney next week lets see what he will say about it. He is charging 600 for consultation only :s I hope I will get some way out.
Good luck to you!