A simpler way to get the most out of a single exposure on a DSLR is to shoot in Raw using
UniWB and by "
exposing to the right." UniWB will give you an accurate on-camera histogram so you won't lose any highlight details. Exposing to the right will give you the most latitude in processing your images.
I started exposing this way for difficult situations and was so happy with the results that I now do it almost exclusively.
I think the advice that you should underexpose for dark scenes is probably relevant only for film. With digital, you want to minimize noise in the shadows by exposing to the right, and this is all the more important if a scene is full of dark areas. You can read more about it
here.
Exposing to the right is independent of the problem I'm describing. I agree with expose to the right, when you have a single dominant element in a scene.
A reflected light meter takes the light from the scene and gives you an exposure based on making the dominant element in the scene 18% grey. So if you've got snow or a wedding dress as a dominant relective element, the meter will tell you to underexpose the scene, relative to proper exposure. You therefore have to add exposure back to make the snow or dress white.
If you have white and black in the same scene and need both exposed properly, you have four options:
1. Expose as per your reflected meter reading; in this case, neither the dress nor the tux will be properly exposed, though in practice, the tux will be pretty close because the white dress reflectance will likely swamp the meter with light.
2. Overexpose relative to your meter reading; you'll get a grey tux, but you can always take exposure out in PP and blend the exposures in PS. This is probably the best method, barring the purchase of an incident light meter (see #4), since taking exposure away does not decrease the S/N ratio.
3. Underexpose relative to your meter reading; you'll get a grey dress, but you can always add exposure later. Might not be a great way to do things because adding exposure decreases the S/N ratio, and this MAY be unacceptable, depending on the ISO you're using.
4. Buy an incident light meter so that you're measuring the light falling on the subjects, rather than the light coming off the subjects. This is the best solution of all; no PP needed to fix either the dress OR the tux, exposure is dead-on, and is based on the illumination of the scene. This is also preferable when using flash; ETTL can be easily fooled by highly reflective objects, leading to underexposure.
Incident light/flash meter = your best friend (for portrait/wedding photography)