I remember being on a photo forum (not this one) a couple of years ago. Someone displayed a very dramatic scene of a river going through a city with really dramatic clouds. I responded (paraphrased) "did you add the clouds"? Crickets. I followed his Flickr link, and saw many more images with very dramatic skies, some eerily similar to the one in the city, but in different locations. After a few days, I responded to my own response and said (paraphrase), "question answered, I took a look at your Flickr feed. Looks like a lot of your photos have dramatic clouds". A couple days later I got a private message from the poster. He said "I added the clouds".
My main beef about adding skies is they may be skies picked from a library which represent conditions that would NEVER be seen in that locale, or that time of year/day/etc. for instance. Now, if you state these are graphic arts, I guess anything goes (I have done collages for year book pages for my daughters for instance, but it was VERY clear these were a collages). I can also understand a COMMERCIAL photographer, say photographing a house, tastefully adding a few clouds to enhance a scene, or a portrait photographer removing zits on a Senior high school portrait. This is commercial activity, and one needs to please the customer. But in general, I personally prefer to be more conservative about adding/removing elements (and generally do not at all).