To be 100% honest if I transit through PVG or PEK I ALWAYS make sure my travel agent has a backup flight if I do NOT have a visa. We had a co worker who went through hell when his flight was cancelled and could not technically enter China (though they finally just kept giving him daily transit visa's while he waited 2 days to get on a new flight)

Now to be on the safe side I tend to just have a Chinese visa regardless; handy also if you are doing a NON China transit and you get a cancellation but can reroute through China