Keep in mind that digital shooting is a linear capture. That means that if you divide the histogram into 5 areas, the highlight area (the 1/5th on the right) contains 1/2 of all the data captured. Then next 1/5th moving left, contains 1/2 of what left and so forth. So if you underexpose, you are moving the histogram left, or throwing away part of the possible data that can be captured. This also means that the shadow areas will be compromised or clipped. So with digital, it is better to always expose to move the histogram as far right as you can without clipping a critical color channel. Keep in mind that most camera histograms still show only the black/white info so you'll only know if you're clipping whites. But in general if you shoot a black/gray/white card at full frame, then you'll be able to adjust the histogram to get three spikes and you'll try to move the spikes as far right as possible without clipping. If you have speculars in the scene, then it is often okay to clip them. As a general rule one should avoid setting the compensation on the camera to a minus value unless the scene really justifies it. Instead use the black/gray/white calibration target to calibrate the scene. You can quickly see exactly what compensation is needed. This is especially true when using the built in meter. With digital, even a 1/3 stop under/over exposure can start to cause problems. And if its under by more than 1/3rd, you can almost guarantee that many shots will have areas that are not recoverable due to the linear nature of digital photography.