I asked this same question a while back and most people advised me to avoid a dinner ADR on the day of the party. Reason being, the restaurants are crowded for early dinners because a) people are booking the early dinners who are staying for the party and b) people who are not staying for the party have booked early dinners so they can eat before 7:00 when the park closes.
However, I don't think the party would have the same effect on the resort restaurants, so if you were going to do one out of the park, I wouldn't be as concerned about the wait.
We are taking a different approach on our party day. We are going to head to MK at 9:00ish the day of the party, maybe hop on a ride or two, then have a brunch at CP at 10:20, and heading back the Poly to rest/swim.
Then, we will return to the park for the party around 4:00 and will do counter service for dinner.
Don't know if my plan would work for you as well, just trying to give you some other ideas and options.