In my opinion postwork can change a mediocre render into an amazing piece of art.
I have been using photoshop at work and at home for the last 15 years, and I would feel empty without using it at some point.

While I understand the believe people have here at Rendo that you should do as much as possible within the render, I think it can be incredibily time consuming.

Learning an image editing package will come useful, you will come up with shortcuts that may reduce your workflow inmensely, and may be able to do things that within the render would have been complicated or my have been had to pay for (textures, shaders, etc.)
Also you may be able to add some personal touches to your renders, such as editing textures, etc.

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-'I curse the day my curiosity led me to investigate the strange stain inside
the atomic acelerator chamber'-
The Nuked Bug