I was happy to find a thoughtful weblog post on one of my favorite subjeccts: At what point do you delete your undesirable images? In his post, To Delete or Not, That Is the Question..., Carl Weese says, "To me it seems prudent to avoid deletion in camera. I also reformat the memory card in-camera, after downloading and making two copies of all the files on either hard drives or optical media. Call it belt'n'suspenders."
Carl discusses additional reasons not to be too delete-happy, even once the images have been loaded on to the computer and examined on the monitor. My personal view on this is to only delete the outright unusable dogs and archive the marginal stuff. A slightly underexposed, badly composed image of Uncle Bob might not mean much today, but 10 years from now could be a valuable, touching memory.