Depends on where you want the major focus to be. On the sunset image, the clouds are stunning and should be give their space. On that nighttime Chicago image, the reflections are impressive and should be prominent, as
dcstep said.
In this pano of Chicago, I wanted the city itself to be the focus and to avoid the artificially induced curve of the straight pier just below the tripod. If the horizon was any higher, you'd see the pier. Any lower and the sky would dominate, which was also not my intent. This is one of the many areas where we digital photogs have it so much better than the film days. We can play around with horizons in post processing, whereas with film you'd have to get it right in the original framing (more or less).