Dean, I believe that your original thought about the Center Width, Panorama and Dimension adjustments is correct. They only apply to adjusting the amounts of 2-channel material extracted and sent to your center or surround speakers in your 5.1 setup. 5.1 material such as DD or DTS tracks on DVDs already have discrete center and side surround channels and there's no need to extract sound from the mains to fill them.
As far as applying them when using 2-channel music sources(I always use DPLII(x) on all 2-channel music sources), I leave the Center Width setting at the default middle if using a center speaker(rather than the phantom I usually apply), adjust Dimension just slightly to send more ambience material to the surrounds, but usually leave it at the default middle and always leave Panorama at the default off, since sending direct sound rather than ambience from the mains to the surrounds is an unattractive "all channel stereo" effect that I don't care for.
