Assuming you are referring to load-bearing roof racks, there are a few things to consider when deciding where to place them.
The most important factor is the distribution of weight. You want to avoid having too much weight on one side or in one area, as this could cause the rack or even the whole vehicle to tip over. If possible, try to keep the weight evenly distributed between the front and back, and between the left and right sides.
Another thing to consider is wind resistance. Placing the rack too far forward or too far back can create a lot of drag and make your car harder to control. Again, try to keep it as close to center as possible.
You also want to be sure that whatever you're carrying on the rack doesn't extend too far beyond the edges of your car. Not only does this increase wind resistance, but it also makes it more likely that something will get caught on something else and tear off. So be sure whatever you're carrying is secured well and doesn't hang over the sides.
Finally, you'll want to think about how easy it is to access whatever you're carrying on the rack. If you have to climb up onto the roof every time you want to get something out, it's goingto be a pain (literally). So try to positionthe rack so that whatever you're carrying is within reach without havingto climb up ontothe roof.