Naming groups of machines in a common way is very popular, and enhances communality while displaying depth of knowledge as well as imagination. Certain sets are finite, such as the seven dwarfs. When you order your first seven computers, keep in mind that you will probably get more next year. Colors will never run out. Use real words.
Random strings are inappropriate for the same reason that they are so useful for passwords.
They are hard to remember. Use real words. Don't worry about reusing someone else's hostname. In all other cases, the local domain is assumed to qualify single-part hostnames. This is similar to the way phone numbers are qualified by an area code when dialed from another area.