You should ensure the "back" speaker is a good timbral match to the surround speakers, because it's signal is matrix-derived from the left and right surround signals. Thus, you want a smooth enveloping mix across the side and rear. You won't lose directional cues,either.

You do not want the "back" speaker to call attention to itself because its tonal qualities don't match the two surround speakers.

Alan Lofft
Axiom Resident Expert

Alan Lofft,
Axiom Resident Expert (Retired)