When I shot film this question never came up. However, your choice of film made a huge difference. My primary film was Fuji Velvia because I loved the rich greens that it provided to my landscape shots. The choice of lab that I used to process the film also made a difference. Someone I didn’t know made decisions about the amount of time the slide film stayed in chemicals which changes the image.
When the shutter is actuated you are no more than half way done. There are stories about how Ansel Adams, one of the greatest landscape photographers of all time, would spend hours in the dark room perfecting a photo only to come out and rip up the image because it did not fit his vision. I think most people just assume that you use Photoshop (now a buzz word for post processing) to make a bad image good. One phrase that I repeat to myself often is “garbage in, garbage out”. It is vital that the image be the best it can be at the time of capture. This limits post processing time and ensures that you have a good product to start with. Photoshop is used to make a good image the best it can be. Generally, not always, if I have to spend more than five minutes on a photograph in Photoshop I just scrap the image.
What is the difference between Photoshop and the traditional darkroom? I say it is the amount of time you spend processing. In the traditional darkroom you could spend hours dodging and burning or playing with the paper exposure times for a single image. It takes a fraction of that time in Photoshop to make the same adjustments.
So, can an image be OVER processed? Yes but it is really up to the photographer’s vision and what he/she is attempting to produce. My goal, in most cases, is to render the image as close to what I remember seeing at the time of capture as possible. I do sometimes stray from this just to see what happens. Photography is art and therefore subject to the vision of the photographer.
What you should ask yourself at the end of all your processing is this question. Do you like what you see? If the answer is yes then it really doesn’t matter how you got to this point. People who purchase your art work generally don’t care because they simply like what they see.
I do have to wonder if traditional artists go through a similar issue? Do you think some painters look down at others because they used mixed media or encaustics? Does their choice of paint brands come under scrutiny by other artists?