Most generally the decision rests with the crew and what they think ATC is going to allow. AWACers are very familiar with these situations being they used to fly out of ORD. If you were ready to go you could usually get out earlier....if you weren't you would continue to wait. I don't know much about BOS but my experience with PHL and LGA show that if there is a ground stop due to WX you can count on the delay going the full monty...If there is a ground stop due to flow...that's subject to change and EDCTs are usually set in stone unless its a nice day.