Well, It is not that common but i have heard it once or twice. (not just in 300). They use the shield as an object that if in your hands you are still defending your coutry (or city-state). And if the owner of the shield has lost the battle then the shield is lost.
BUt as they use to burn the dead the ashes were placed in a jar and buried. (Not actually on his shield or even with his shield).
It is more a metaphor of saying this is what protects you from life and death.