Your skies will always profit from some preparation beforehand. In most renderings, the sky should enhance the building features without trying to dominate, and help to highlight the center of interest, particularly in those features that extend up into the sky, such as the tall buildings, trees, and so on.
The positioning of elements such as large cloud masses, the brightest part of the sky, or the most colorful, can considerably influence the overall composition of the rendering. On the other hand, you may wish to play down an area, most commonly the utmost point of the rendering.
In the above image, where is your eye drawn? because of the sky composition, your eye is drawn to the bottom, left 3rd, of the photo.