Uncomposted wood will tie up nitrogen as it decomposes. I am not certain on the cedar being phytotoxic. It is resistant to being rotted, but I don't *think* it will directly harm plants. It and the oak will tie up N if mixed into the soil though.

Cedar sawdust is fabulous for acid-loving plants like blueberries and azaleas. My father was an avid amateur woodworker and used a lot of it when he was alive. I used to take big garbagecans full of cedar sawdust and cedar shavings for my blueberries. They thrived. Made the best-smelling mulch around.

I'm sure cedar wouldn't be phytotoxic like, say, walnut, since they sell TONS of cedar mulch.

When I took my Master Gardener training in RI, I was too embarrassed to admit I added wood mulch to my soil to loosen it up - till the instructors offered that that was what they did. No details on which woods were better or worse - I used mostly pine in NY and in NM I can only get cedar - it's just a matter of what I can get in the home centers. Soil is quite alkaline here, and no adverse reactions to the cedar.