Too bad the 950 manual provides only a vague description of both 5-stereo and 7-stereo. To the best of my knowledge, the low-down is this:

In 5-stereo or 7-stereo, all left channels carry the original left channel signal, all right channels carry the original right channel signal, and the center channel carries a signal much like the 950 would provide if you were in the Pro-Logic II mode. Crossover and calibration/trim level settings for your various channels still apply.

I suppose these stereo modes, if your surround speakers can handle about as much power as your mains, are the way to either host the loudest party you can or to just shake the cobwebs anywhere in your home, or on your person, loose. For the curious, it is also a way to find out just how much your amp can put out if all the channels are driven with full signals available.