The best time to throw an image away is before you take it. This is sort of hard, though, since we tend to fall in love with subjects and ideas, and we try them out. In the age of digital photography, the cost of failure is very low, so we shoot and shoot. At least, I shoot and shoot and shoot.
Your ratios will vary, but if you're not throwing things out pretty aggressively, either:
- You are awesome
- You are not throwing away nearly as much as you ought
- You are not shooting enough
If you're shooting fast action (e.g. kids) with a digital camera, when your goal is to make a pleasing record of a family event, you might throw away 90% or 95% of what you shoot. On the one hand, your standards are pretty low so you might keep a lot, but on the other hand you're shooting digital so you're blasting away pretty vigorously to try to get at least one good image of each important moment.
If you're shooting something quick, but for art's sake? Let's say you're shooting a portfolio of fine art photographs of hockey. You might shoot a 100s of images, and keep 1, or none. Your standards are high. The action is fast and unpredictable. Perhaps you're shooting digital so each exposure is free.
The point is, you should always be editing. Ruthlessly and thoroughly. Don't accept a bad image, or a dubious one. Shoot enough frames to get what you want. Go back and re-shoot before you try to save a bad image. If the image doesn't look good in a thumbnail or on a contact sheet, it's probably not going to be worth a damn when you're done with whatever your process is. Ruthlessly refuse to press the shutter button if the image is crap, no matter how much you love the subject. Ruthlessly delete the out of focus images, no matter how pretty the out-of-focus girl is. Ruthlessly trash the 10 lousy shots and only even consider the 1 best one of the set of 11 you shot of that thing. Then throw that one out too, when you realize that the light post is sticking out of the model's head.
Don't try to save a bad image, unless there's no other choice. HDR isn't going to make a bad image into a good one. Punching the contrast through the roof won't either. Oversharpening it? Guess... just guess. NO! NO IT WILL NOT! Sometimes we fall in love with these terrible hot messes, and we spend way too much time trying to make something good out of it. Usually there is something good in there that we've fallen in love with, but if the image is fundamentally bad it's because the badness overwhelms the good. You can't parse the badness away, no matter how you try.
If you ever do manage to save a bad image and turn it into a good one, it's likely because you're a better painter than you are a photographer. Perhaps you should consider a career change?