When choosing paint, should young families always favor an eggshell finish because it's easiest to clean?
Also, are bathrooms and kitchens exceptions to this rule because water vapor can cling to the walls?
I would say that it depends on the type/brand of paint you are using. We have used eggshell finish paints from Benjamin Moore and Behr and it used to be true that eggshell was the easiest to clean. However, newer paint such Benjamin Moore's Aura is very easy to clean, especially their Bath & Spa paint. It's a kitchen & bath paint, but it has a matte finish to it. It looks great and is easy to clean, yet more durable than the older kitchen & bath semi-glossy paints.