To optimize QEMU using, we need to set cache settings on the QEMU command line, and the QEMU command line settings override the Ceph configuration file settings.
If I set the qemu caching parameter in nova.conf by,
      disk_cachemodes="network=writeback"
This would give me "cache=writeback" in the qemu cmd argument for RBD device when VM is created. According to the ceph doc above,  It would be equivalent to setting of "rbd_cache = true". 
This is recommendations from Ceph documentation and from best practices.

Sources:
* http://ceph.com/docs/master/rbd/qemu-rbd/#running-qemu-with-rbd
* http://www.sebastien-han.fr/blog/2013/08/22/configure-rbd-caching-on-nova/

Obviously, this recommendation is not applied when not using Ceph to store instances, that's why we made differentiation.