I use one "rule" that works for every image. Never overexpose any important highlights. I am not talking about rim light when you are shooting a subject that is backlit, or specular highlight reflections on water. But if you are shooting a bride in a white dress, next to a white wedding cake and the groom dressed in a black tux, make sure you expose the cake and the dress correctly.

Your camera's meter is immensely helpful, especially if you use it's spot meter function.

In the above example, I would point the spot meter at the white dress, and use a camera setting that adds 2 stops (or more if your camera has more highlight "headroom") more exposure. This would allow the most amount of exposure without losing the details/texture in the dress and the cake.

This would be the opposite of what I would do with a hand held spot meter used with negative black and white film. I would find the darkest feature that I wanted to retain detail in, and adjust the meter's suggested exposure settings for two to three stops less light. This would ensure that my shadows would not be completely black. Manipulating developer temperature, concentration and "time in the soup" would help reveal the shadows (thin areas on the negative), while suppressing the development of the highlights, for a more balanced negative.

This works for any scenario. Post processing can always reveal shadows, but overexposed highlights are typically unrecoverable.

In addition to the spot meter, you have two more tools - the overexposed highlight indicator, or "blinkies" which will reveal areas that are overexposed as blinking, and the image's histogram. Keep in mind that if you are recording images as raw files (highly recommended for maximizing the dynamic range of an image) these two tools use the camera-generated preview, which is based on a jpeg version of the image, and will have narrower dynamic range. When you look at the histogram of the image in your processing program, if you maximized your exposure based on the in camera histogram/blinkies you'll find that you might have been able to use a little more exposure without risking overexposed highlights.

Hope this helps.