Placecards question

Does anyone know what the standard 'address' is for placecards? Is it just, 'Mr. and Mrs. Smith,' or 'Mr. and Mrs. John Smith.' Because a lot of people at weddings have the same last names, I'm thinking that you include first names to avoid confusion, but do you do that for everyone, or just for the people who share a last name with another guest(s)?

Thanks!