There is a school of thought that says "photography should remain pure"; that is to say that it should remain free of unnecessary image manipulation. This, of course, is directly aimed at the use of Photoshop and other post-processing software.
There was a point in my life where I subscribed to that thought as well. It was particularly useful when I was learning the basics. The assumption was: if I can select the right subject, compose the image and get the proper exposure then I have a technically "good" photograph. Needless to say, I don't subscribe to the "purist" philosophy any longer - though I still think it is important for every photographer to be able to take a technically "good" photograph with as little assistance from the on-board electronics as possible. Try it. Put your camera in "Manual" mode and turn off auto focus. Now select an image (outdoors in daylight works best here), set your ISO speed, shutter and aperture for what you think is the correct exposure and then manually focus and take the shot. If you need some tips, see my earlier blog on "Making Light Work".
Okay, so how did you do? Was your image good enough to print directly from the camera? The truth is that technology has made getting the "technically good" image much easier over the years. Multi-point auto focus, built-in multi-zone metering, image stabilization; all of these things make it faster and easier to take pictures.
But all of that is still pre-processing - it happens before the image is taken. In digital cameras, the camera actually can (and usually does) go a step further. It automatically adjusts the image tone (brightness, contrast, color) when it saves the image to the memory card. So it's actually doing some basic post-processing for you - the same kinds of things that you can do in Photoshop! Professional photographers usually avoid that in-camera post-processing by having their camera save its images in "raw" format. This format varies from manufacturer to manufacturer but it basically saves all available image data and creates a very large image file in the process. The good news is that the post-processing software used by the photographer can leverage all of that information to help create the best possible image.
When using a film camera, photographers often did their own development processing so they could "adjust" the image in case the working negative was underexposed or overexposed. When they printed the image, they could also select different types of papers and chemicals to give the image a particular "look". Again, a lot like what is done now in Photoshop for digital images.
Post-processing is as much a tool as the camera itself and, now, with so many options for adjusting color temperature, saturation, sharpness, tone mapping and HDR processing it is possible to take your images in completely new directions - and different directions. Personally, I use Photoshop to remove annoying and obtrusive bits of trash, branches or telephone lines and then i use OnOne to get the stylization I want. Often I will use PhotoMatix to blend multiple exposures into an HDR image and I will also use Topaz for additional stylizing. All of these tools allow me to get the look I want and to capture the viewer's attention just long enough to go "wow!" or "cool!". Of course it doesn't hurt to have that "technically good" image the "purist" in us strives for to begin with.
The bottom line is that each of your images is, potentially, a piece of art. How you manipulate the image to evoke the desired viewer response is as much a matter of personal taste as a artist's selection of brush, canvas or technique. And, like any artist, your goal is to have someone appreciate the image so much that they buy it and hang it in their home or office.
All of that said, there are times and situations where image manipulation can go too far. A case in point is Journalism. Manipulating a journalistic image to depict what was not there or to unduly enhance negative (or positive) aspects of a person, event of situation is dishonest and, potentially, illegal.
In my opinion, image manipulation is as necessary as "developing" a negative or print used to be years ago. It is up to you, as the photographer, just how much you think the image should be manipulated.