Cedar
Conifers, should not be used for smoking
Resinous woods such as conifers create smoke thick with unburned carbon and pitch.
Cedar is a conifer, Conifers should not be used for smoking. However Cedar planks are safe to smoke or grill on.
Planks for grilling, untreated cedar, Hickory, alder, or maple.
Sassafras
My father has smoked with Sassafras many years ago, he said he used to chew on the twigs as a kid and from what I read a sassafras twigs stimulates saliva production: a useful fact for desperately thirsty hikers.
The dried and ground leaves from sassafras are used to make filé powder, an ingredient used in some types of gumbo.
In 1960, the FDA banned the use of sassafras oil and safrole in commercially mass produced foods.
Safrole oil is extracted from the root-bark or the fruit of sassafras plants.
The roots of Sassafras was used in the flavoring of traditional root beer.
Sarsaparilla, sassafras are ingredients still used by hobby or microbrew enthusiasts.
Sassafras wood is commercially available for smoking, however, smoking woods are not regulated.
Would I use Sassafras wood for smoking? If I had sassafras, absolutely, after all my research I have not found any reason not to use it.
So I have to concur with Cliffcarter on both points.