So according to the Cedar Shake Bureau you're not supposed to seal any cedar shake after cleaning it because it can't breathe and allow moisture to naturally evaporate. Instead you're supposed to clean it as usual but apply a penetraiting oil finish rather than sealant. This is supposed to keep the wood moisturized and healthy. Seems like everyone's sealing them instead. Any thoughts? Here's a link to the page where I read about it.
http://www.cedarbureau.org/faq/maintenance.htm
http://www.cedarbureau.org/faq/maintenance.htm