I am not so sure Estes really did a thorough job of proof-reading here, or that the cardboard actually needs to be sanded.
If there is any sanding necessary at all it would be on the root edge, to be sure that it is nice and square (not punched at some goofy angle by a machine). Other than that I would not recommend sanding (yes, I guess I am contradicting the manufacturer's instructions).
Hammer's suggestion to reinforce edges with CA is a pretty good one, AS LONG AS YOU ONLY DO THE OUTER EDGES. Don't put CA onto the root edge as this is where you need your white (or yellow...or brown) glue to penetrate the fin material for a good structural joint.
Everyone has a right to be stupid, some people abuse the privilege