My motto - if the postman delivers, it must have been okay.
When people do or don't take spouses names, it can get confusing. I typically address Christmas cards to X and Y Smith. If I know differently, it will go that way. If I don't know that Darla's pal, Otto's mom uses a different last name, it's going to X, Y, and Otto Smith. Sure, I want to send cards correctly, but I don't really see what the big deal is if it's an honest mistake.