This question came up at my girlfriends studio the other day. I choose a letter based on what the intended person identifies with the most. Some people have very unusual or dominate first names, like Zenana, so I choose a Z for her. But my family really identifies with our last name even tho we have new ones after marriage, so it is the first letter of the last (maiden) name for some. Of course, nowadays, shabby chic decor doesn't require you belong to the initials displayed in your home, they are just a fashion statement. That would not work in the White House, or the ancient European families, but for most everyone else, they just want it to be decorative and beautiful.