I would be there at 5:00 a.m. Your ticket counter may not be open till around 5:30 but at least I am already in line so that I can be one of the first ones to check my luggage in. That way I am not rushed to get through the security line in order to make it to my gate.
Remember 7:00 a.m. means they will start boarding your flight as early as 20-30 minutes before the flight time. So they will begin boarding around 6:30. If the ticket counter opens at 5:30 that only gives you an hour to check the luggage and get through security.