The majority of photos needs a touch up (unless you know what you are doing or are Amazon of course with the patent for photographing an object against a white background. It needs no touching up.)
I took the photo below which was more for the record of the area rather than get a great photo. I probably would have gone at a different time of day to try and achieve better light if I had time. It’s underexposed (deliberately) and the sky was a wash out.
I decided I would replace the sky…. I’ve never been keen on this because I think it detracts from the effort that a photographer puts in to get the perfect shot. If you’ve only got one chance to get a photo though, should it be okay to put in a new sky? Probably yes as long as you don’t enter the photo into a competition. I used a combination of Photoshop and Lightroom to alter the photo.