I received issue #30 of Chip Chat yesterday, and on page 1 is an article by Doug Saito that should be required reading for everyone in the hobby. It is entitled "To Clean Or Not To Clean... A Chip"

As many of you know, Doug has always been an advocate of cleaning EVERY chip and here he outlines all of the reasons to do so. In addition to all of the obvious reasons, such as health considerations, the fact that sometimes conditional defects are obscured by dirt (including drill holes), and the fact that clean chips sell better, Doug also mentions something that relates to some of the recent threads here on The Chip Board...the fact that dirt can produce a perceived color variant when none exists.

If you're not a subscriber to Chip Chat, you certainly should be. It's probably the best educational and entertaining periodical in the hobby.