I heard two opposite opinions in the past few years whether it is better to overexpose or underexpose pictures. Obviously, the best condition is when you expose a picture correctly, but you might find yourself in a situation when you might need to underexpose or overexpose a picture and you will have to choose which one is better: to underexpose or overexpose. I heard Scott Bourne (at least I'm pretty sure it was him) saying that you should underexpose by one stop to 'naturally' saturate the colors already in a camera - this apparently saves you some time in post-processing. However, this tip is more applicable for film photography rather than digital photography, because saturating the colors in post-processing can be done nowadays in a matter of few seconds.
My preference is to overexpose when I have to, instead of underexposing pictures. The reason is quite simple and it has to do with the amount of data DSLR can gather and save in RAW files these days (and I just realized that I should have mentioned at the beginning that we talk here about pictures shoot in RAW, which retain much more data and information than JPG files - this is really helpful in post-processing). So why do I overexpose when I have to? If you underexpose and you want to brighten a picture or a part of it in the post-processing, you might find yourself having noise in the underexposed area. Overexposing doesn't create that noise effect and even thought a picture may seem really blown out if you overexpose, a camera captures much more detail in those overexposed areas than you think - this is the magic of RAW files. Therefore, when you darken a picture or parts of it, you will still see much details in the previously overexposed areas.