When should I add an attribute to the Active Directory index ?

When should I add an attribute to the Active Directory index ?

How do I know if I should add an index to Active Directory and if so, how ?

This Blog entry explains it very well.
http://blogs.technet.com/ad/archive/2008/04/01/how-to-create-a-mosiac-of-user-thumbnails-in-aduc-dsa-msc.aspx