Spence, it be the answer in my view that since the vast majority of the material reproduced by the side and back surrounds is rather ambient in nature, a more dispersed rather than sharply focused presentation is advantageous. The surrounds therefore shouldn't be aimed directly at the listener.

As to their placement in the room, your arrangement sounds good, but to comment on a point John B. mentioned, with actual back surround speakers present, no help from the side surrounds in forming a rear sound field is needed. The sides don't need to be slightly farther back as is suggested for a 5.1 setup. They should be directly to the side if possible, or even slightly forward if it isn't. Of course very small variations in this aren't critical.

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Enjoy the music, not the equipment.