If your going for a specific themed Army just use any symbol that can match out the theme (the joy of
40K...their is no "Right Way" of paint schemes or symbology).
For my ogryns (Japanese Themed Army) I used a Samurai Family Mon (crest) just for the ogryn (each of my units has a unique family Mon).
I've also seen people use the vehicle numbers decal and put them on the back of the ogryn like a tatoo.