Daniel Rumbell The appendix has some information that might be useful for keeping various groups and their overall purpose in mind, but you don't need to read any of it. The novel covers the information very well on its own, and, frankly, the appendix is very boring.
GS I highly recommend reading it! But AFTER you read the book. I found all the appendix chapters fascinating, with critical information on the dune universe. Reading it at the beginning would act as a spoiler though.
James Elkins "Need" to read? No. But the appendix does provide added worthwhile information and perspective should you choose to read subsequent books in the series.