Up until recently I've reserved multichannel playback only for multichannel sources. If it's encoded in stereo, then I used only my main LR + sub as a strict rule. My AVR has been in its "Auto" surround mode for years.
However, I've recently taken to using Dolby PLII on stereo cable TV shows. Virtual surround lets me emphasize dialog while diminishing the accompaniment. More often than not, unless my daughter is watching cartoons, my cable box is tuned to PBS. I DVR mostly news shows -- PBS News Hour, Charlie Rose, John McLaughlin (RIP
), Gwen Ifill's Washington Week, and so on; and when browsing the channel guide I tend to stop at car shows, cooking competitions, home improvement shows (but not with tiny houses!), How It's Made, and other such non-fiction stuff where I might learn something. Those programs are all encoded in stereo. Not that I don't enjoy Game of Thrones, Sherlock, and a few other fiction programs, though. In any case, speech intelligibility outweighs musical audiophilia for my casual television watching, so the center channel is usually on via Dolby PLII for stereo TV shows. And since my daughter is usually in bed at 8:30, the center channel is a nice way to boost dialog while de-emphasizing explosions.
But for stereo music, I still prefer 2.1 in almost all cases.