I'd always been told shadows are NOT the preferred target, because after a point, black is black, and it is very easy to have a wide range of exposures deliver maximum black - ie anything beyond a certain threshold will still deliver DMax in a shadow area, but the lowest possible exposure to deliver DMax may render an area you want middle gray somewhere between Zone V and Zone VII. Same with a highlight - if a highlight is blown out, you may still be getting paper white in that highlight when you've pushed a middle-gray skin tone down into Zone II.5. Better to pick an area with medium density as the target. Then if you've done a good job exposing and processing your film, you'll be less likely to be fooled.