I really liked this author's point of view on this (and a number of other things):
Appendixes or appendices – and who cares anyway?
You can follow his (her?) link to another forum if you wish.
I'll summarize: appendixes has been used for a few hundred years and no one will misunderstand what you mean, but there are few mistaken prescriptivists who insist that only appendices is correct and you will cause them to (mistakenly) think you've made an error if you use the perfectly correct appendixes. So choose the one you want, but be aware that there are idiots out there who will try to "correct" you.