That is a GREAT question. I run two center channel speakers, one below the screen and one above the screen (top center), you can see the pic in my profile.
I would NOT do that with your two left over book shelf speakers though.
As for the BEST use of those two left over book shelf channels I would recommend using them as wide channels (again, see the pic in my system of the wide channels). You need to set them at a 60 degree angle to your MLP and you will love it. Make sure the tweeters are the same height as your front channels.
FYI
The extra channels are called Left and Right Wide, Height, and Back Surround, and each requires its own speaker. The Wide speakers should be located at ±60 degrees from the center—that is, well outside the main L/R speakers but still in front of the listener. The Height speakers should also be located outside the main L/Rs at an angle of ±45 degrees, and they should be raised above the plane of the other main speakers at a vertical angle of 45 degrees. The placement of the two Back Surround speakers is not well defined, but they should be directly behind the listeners.
Why add these channels? Audyssey's extensive research clearly indicates that human perception of directionality is much more precise in front of the listener than behind. Thus, expanding 5.1 to 7.1 by adding back-surround speakers is not nearly as effective as adding speakers in the front. According to Audyssey's findings, adding the Wide speakers is the most effective at increasing the sense of envelopment, followed by the Height speakers. Adding Back Surround speakers is the least effective because our sense of directionality is relatively poor to the rear—in fact, a single Back Surround speaker works perfectly well.
A demonstration this week at Audyssey's facilities in Los Angeles definitely showed the effectiveness of the system.