Given that Yorkie referenced the original writers in the first place, it is fair that we discuss the original fluff- as the aforementioned writers intended it to be. Here is a screen grab from 'Ere We Go':
http://i797.photobucket.com/albums/y...pstcbo5wrj.png
Where you can clearly see the word caste being used. Orks have been a caste based society in the minds of the authors right from the start.
In fact the system of tribe/clan/caste is directly analogous to the Tau system of septs and caste.