Let me start by saying that I have absolutely no experience with cedar.
Cedar has some oils in it, that can be toxic to chickens. This is more of a risk if you're using cedar shavings as a bedding material, as the surface area allowing evaporation would be astronomically larger than with solid pieces of wood. You might want to clear away sawdust residue from cutting the posts to size, but unless your chickens are going to be gnawing on the posts 24 hours a day, I doubt there is any risk involved with using them. I would take similar precautions as when working with treated wood. And I used treated wood in my coop without worrying about it at all, I just cleared away the sawdust.