The one monument that depcits the greatest number of shield design is Trajan's Column. The sculpture was created at the very hight of the Roman Empire and its army. The shields on the column represent a very specific time in Roman history, but their symbolism could be applied to anytime in early to mid imperial history. Lino Rossi, a modern historian, conveyed his ideas on the symbolism in his now out of print book, Trajan's Column and the Dacian Wars. I present a streamlined version of his concept here:
The wings and thunderbolt design was almost never seen on the non-citizen auxiliary shields. The few times that it did appear could be explained by attributing those instance to auxiliaries who were awarded citizenship or those units who were not citizens, but served the emperor directly. (such as the emperor's bodyguard)
The wreath design is very common on auxiliary shields on Trajan's Column. A wreath (or cown) was awarded to a general who retuned from battle with victory. There were also a number of military decorations awarded as crowns. It can be guessed that whe this design was painted on a shield it meant that particular unit had recieved an award for some great victory.
1. Greece and Rome at War, by Peter Connolly.
2. Rossi, Lino Trajan's Column and the Dacian Wars.