"Exposing to the right" is now common wisdom when you're setting the exposure for a digital photo. (The right being the histogram's right side where your exposure's brightest/lightest portions are displayed.) This standard advice says that your histogram should skew as far right as possible—without wiping out details in any areas except the scene's pure whites.
But it's really tough hitting that perfect exposure point where only the utterly white whites are blown out (as in this example). That's especially true in the field when making shots and checking them on the back of your camera (even using a scope). For one thing, the little histogram on the camera screen isn't nearly as accurate as the one Lightroom generates on your computer. The difficulty's compounded when you're shooting raw files since even raw-capable cameras display their previews as JPEGs.
As a former slide film shooter, I tend to play it safe by making a slightly darker exposure to make sure I don't lose highlight details. Expose-to-the-right champions contend, however, that that approach has its own problems. If your photo comes out too dark and needs to be lightened, they say, you wind up adding a lot of noise into the photo. The right-is-right crowd maintains that it's better to risk a bit of over exposure and darken the photo in post-processing since that generates less digital noise.
The Online Photographer's regular columnist Ctein has jumped right into this thicket of left and right wingers by calling expose-to-the-right "bull." In an admittedly provocative post, he goes on to call expose-to-the-right advice "dangerous" since you run the risk of losing crucial highlight details that no amount of post-processing can bring back. As for warnings about the noise generated by post-processing lightening, Ctein says such fears are overblown.
He backs up them fightin' words with some pretty convincing example photos that show how much detail can be lost by pushing exposures to the right. In the end, he suggests a method not far from my own: exposing a bit left of right. Definitely worth reading, and making some test shots of your own.