I often use DTS Neo:6 Music for CD's or radio if I want to engage the surround speakers. It tends to give you a wider sound stage with more mid-bass than ProLogic IIX. Prologic IIX tends to give you a more defined center than Neo 6 Movie mode so it often makes dialogue more intelligible.
I do vary from these norms a lot though depending on the source material.
I don't use Stereo 7 for critical listening, but it's the ultimate party mode for supplying music for multiple small conversation groups scattered about the room.
I fully agree with the others for stereo, the upsample is great.
Like most everything else in this hobby, it all boils down to trying all of the options and picking what you like for the particular recording and situation .