A bench with a slight incline (about 15 degrees) is most effective. This places the sternum parallel with the floor, so in a sense it makes it a true "flat" bench press, whereas a traditional flat bench is more accurate a decline.
One tidbit of note: You cannot emphasize the upper pecs (clavicular portion) over the lower (sternal). You can only de-emphasize. In other words, the incline press activates the lower pecs less, and the upper pecs just as much as a flat press.
Perhaps in theory, by sacraficing the development of the lower pecs, you could give the illusion of a bigger upper chest, but who knows. Most of the time that you see advanced bodybuilders with big upper chests, you're looking at the results of testosterone, which gives preferential growth to the shoulder girdle musculature. Of course most pros simply say "inclines for upper chest!" and so it stuck. Interesting how rumors in the gym perpetuate.