Yeah the saps can be toxic or have antibacterial properties, I believe cedar has antifungal/bacterial properties that why they say not to use it. After reading what Tom Barr has said about old driftwood cedar I decided to give it a shot, turns out my water parameters are fine and no fish die offs so the saps are probably gone.
My mom has a piece of petrified wood which would be cool but I'm not sure what long time exposure to water would do.
You can drip some vinegar or acid solution on the petrified wood and as long as it doesn't fizz it should be fine to use in your tank. If it fizzes it will more than likely mess with your PH and should be avoided.