Agree with Eric here. Projected images will benefit from a 2 pixel border denoting the image boundary on an otherwise big screen with no idea of the composition. I will often colour key the border to a predominant tone in the image, although plain white or a grey tone will do the job provided the border is not too thick and become distracting.
If you display images on line, the background colour will often be white, so a hi key image will again loose its boundary and composition. Some hosts will offer differing background colours but not usually changable for different images - more a blanket choice across your web space, so again a border can be beneficial where an image is loosing all or some of its edges.