Reconsider the two rules for the plural morpheme /z/:
a. Insert a [?] before the plural morpheme /z/ when a regular noun ends in a sibilant, giving [?z].
b. Change the plural morpheme /z/ to a voiceless [s] when preceded by a voiceless sound.
Reformulate these two rules so that their order of application doesn't matter. (This shows that the necessity for rule ordering depends on how the rules are formulated, but that if we make the rules very specific to avoid rule ordering, we may sacrifice a degree of simplicity.) How is your reformulation somehow less simple than the one that requires rule ordering?