There was a time I could name on sight every insignia for every rank in the Navy from Seaman Apprentice to Admiral.

Too many things to remember though, one symbol meant one thing in one color, something else in a different color, and could be confused with a job position that had the same name. (Like Captain, for example. It's both a rank and a position.)
It's not necessarily important for writers though, if you don't want to work one out. It'll really only add a small touch of detail, unless the plot makes them important enough to actually explain. It could definitely be fun though, working out the symbolism that represents each rank.
I might do this in my sci-fi projects, since those are generally more military oriented than any of my fantasy books.