Does Traffic Shaping & Policing take into account L2 overhead? (Cisco IOS)

I thought I read somewhere modern Cisco routers take the L2 overhead into account when you configure traffic shaping, but our AT&T sales engineer is telling me that you have to configure traffic shaping to something lower than the CIR to compensate for the L2 overhead.

Is this true? Or is AT&T just trying to cheat me out of 10% of my bandwidth...