As a former retoucher I generally see the need for every single image to be retouched to some degree. There are so many subtle things that can be done to improve the attractiveness of an image that it just seems silly to me to not take advantage of it. However, if you were to pin me down and list the five projects that need to be retouched no matter what, I'd give you the following examples.
Five Types of Projects that Need Retouching
1. Food Photography
Stylists, photographers, and assistants all do amazing work with food photography, but when it comes to adjusting tiny details in the food (remove those two seeds, add more bubbles, remove the dimple in the cheese) as well as adjusting the food to achieve the most attractive colors, it's most efficient to do it in post.
CGI is a fantastic tool to use to create imagery, but there are simply times when it is most effective to adjust things in post. Imagine that after every small adjustment in Photoshop you had to wait two hours to see what it looks like. That's what retouching is like using CGI programs. Do it in Photoshop and save time and money. Additionally, retouching CGI images to add flaws into the products make it look real and is much easier to do in retouching. Learn more about CGI retouching in our post on retouching in CGI vs. Photoshop.
3. Color Changes, Color Matching
There is simply no way to completely control color on set to a degree where you are hitting a very specific color. This really can only be done in post.
This is kind of a no brainer - it's much cheaper to photoshop a UFO into your shot rather than build it and photograph it. When your ideas are bigger than your budgets, retouching is the tool that can make it all happen at a fraction of the cost.
5. Shiny Objects
Shiny objects reflect everything from dust to the camera itself. It is always advantageous to clean it up in post so the shiny object does not become distracting and take away from what you want people to be looking at in your image.