One of my reasons for putting a world right next to Mecatol Rex in the last game was to provide a legitimate reason to be near Mecatol Rex without immediately giving away that it might be my secret objective.
I'm willing to try it both ways. I do see merit to doing it completely random.
I've noticed that people tend to leave the ring around Mecatol Rex completely free of worlds. I think this has the side effect of giving away anyone who's got an objective of Mectal Rex. As soon as they move closer to it, everyone thinks that's their secret objective.
One of my reasons for putting a world right next to Mecatol Rex in the last game was to provide a legitimate reason to be near Mecatol Rex without immediately giving away that it might be my secret objective.
In a random placement game, more planets might be near Mecatol Rex, and so the fighting near the center of the map might get very energetic.