I've read and understand Michael's argument for exposing to the right. I can't disagree in general but there are some counterarguments including the matter of color shift that can occur when overexposing. I certainly noticed some of that when I owned a 5D2 and I attempted to push exposure a half or full EV.
There are also some sensors available, including the new 16 meg manufactured by SONY, used in various SONY, Nikon and Pentax models, that are extremely clean in shadow and dark areas, that invite the opposite. Based on my experience with a Pentax K-5 at ISO 80, I can underexpose a full stop and using LR, can bring up fill light, adjust levels, brightness and contrast, and wind up with a far more pleasing image than going the other direction by overexposing. Although I can't push the underexposure quite as far, I've found I can go - half an EV with my 645D and get a better result than by overexposing a half EV.
I would be interested in learning what others here have found in this regard.