This topic has been raised recently with the misconception that post-processing only began with the invention of Photoshop and other image manipulation programmes. However, this is not the truth. Simply looking at black & white photography there were myriad of ways to alter the image in the darkroom: from the type of developer and paper used, dodging and burning, retouching and colouring, and even as far as to alter the negative by scratching. When it comes to colour photography these are also the case, but you can also add the changes to colour here by adding or subtracting yellow, magenta or cyan via the enlarger head.
Today, in the digital era, it is best to shoot in RAW which can give quite bland results. Therefore, there is a need to adjust the images in post. I generally use three main controls in Photoshop: Levels, which will increase/decrease the dynamic range to its optimal size; Curves, which will get me the contrast I want; and Hue/Saturation, which enables me to control the colour of the image. Obviously, these do not take into account and dodging and burning or other effects, but are the basis of making the overall image look right.
For those who say they get the image right when they shoot - in the camera - are most likely shooting in jpeg, which means the camera processes the image, so an automatic process rather than manual. Post-processing is a part of photography and is here to stay, and does not mean that you are a bad photographer if you have to 'correct' your images.
My only concern is when images are manipulated to look differently from the original by adding two images together, erasing flawed backgrounds, etc. Although I have no issue with this use of technology I somehow think it is no longer photography, but rather 'photo-graphics'. Then again Jerry Uelsmann was doing this sort of thing way back in the 60's in the darkroom long before the digital era.