The only photos I usually delete when I'm using my digital camera are those with shutter blur (I moved, it moved, whatever), or if I realized the exposure was way off, etc. But I'm a film photographer, there is no tossing (unless you somehow mess up an entire roll). So there are photos I may not use in any particular way, or may not print, etc. but I keep the originals.
Sometimes if a photo doesn't quite turn out the way you wanted, looking at it later you might see what didn't work and what else you could try.