I always find it interesting when I arrive at a venue I've never played before and find that the DJ booth is nowhere near the dance floor. Sometimes this is unavoidable; for instance, here in the New England area, there are many old mansions/estates that have been converted to function halls, and were not really designed for a DJ to perform. In such cases, I'm sometimes set up in a corner where I'm at a distance from the dance floor, and even in a few places, in a different room or on a balcony overlooking the dance area!
Ideally, the DJ should be placed dead center in the room so that the sound is evenly distributed. This is not in an attempt to be the center of attention, though. It's only for the best distribution of sound, both when it comes to music, and when people are speaking on the microphone.
If that's not possible, then the DJ should at least be along the dance floor. When the DJ is not near the dance floor, what happens is that the music needs to travel further, sometimes among guests tables when they're trying to eat and socialize. Having to yell over the music -- so that it reaches people on the other side of the room -- is not the goal of any quality DJ. We want it to be something you hear but doesn't get in your way.
And that's when the DJ is on the same level as the dance floor. In some cases (although it's likely rare), I've been placed on a balcony overlooking the dance floor. If this is your setup, check with your DJ if he has speakers that amplify downwards (be warned, many speakers do not have this feature). You should also check with your venue that there are no obstructions for the DJ's speakers, such as poles that run along the balcony, blocking sound from traveling. In some cases, a DJ will place one speaker on the lower level and one on the upper level, thinking that that will take care of the problem, but in fact what happens is that the sound is traveling at different distances from each speaker and therefore may sound slightly off/delayed.
Make sure your DJ booth is located for prime performance. After all, you paid for it, why not get the quality you deserve?