Our chickens often roost on a fallen cedar tree in one of their rotating pastures and we've never had any problems.
Here's a blog post on that particular pasture after one year.
http://www.avianaquamiser.com/posts/One-year-old_forest_pasture/
I can see where cedar shavings might be an issue for chickens in an enclosed space, but we've never had a problem with chickens roosting on cedar trees.
A lot of people will even say the cedar helps to keep mites away.
Here's a blog post on one of our rotating chicken pastures that has had a...