You can use either a match for your surrounds, or any direct radiator. A center channel is probably overkill.

For example, people running QS8s for the L and R surrounds will pick either a third QS8 for the back surround, or a single M2, which is direct-radiating but incorporates the same drivers.

As far as I can tell, the jury is still out as to which is better in back, direct-radiating or diffuse (bipole/dipole/quadpole).