There have been some theories bouncing around a few of the photography forums on the Web that claim that you should underexpose by a stop for digital photography.
First off, let me say this: your goal (my goal, our common goal) is to get the proper exposure. That's our goal. Always. But if that's not possible, if given a choice between overexposing (a photo that's a bit too light) and underexposing (a photo that's a bit too dark), go with overexposing you'll have less noise.
That's because noise is most prevalent in the shadows, and if you have to lighten an underexposed photo, in Photoshop (see tip below) you're lightening (increasing) the noise in the photo. That's why it's better to shoot lighter (overexposed), because darkening a photo doesn't increase noise the way lightening it does.
So, if you'd rather have one than the otheroverexpose (but again, or goal is to do neither. That's why we bought these fancy cameras with their highly advanced metering systems).