This is simple physics: if the iMac is at the end of an 18" cantilever, and the iMac weighs 30 pounds, it's going to be creating 45 foot-pounds of torque. Add in the weight of the arm itself, plus the weight of that part of the desk, and you're probably up around 70 foot-pounds. Now double or triple that, to get a good safety margin -- you don't want to put a cup of coffee down on your desk and have your iMac come crashing down. So, the question is now, "Does the part of the desk on the other side of the pivot have enough weight to counteract 150-200 foot pounds of torque?" Unless your desk is made of marble, the answer is probably no, so you'll want to put enough weight on it (or attach it to enough weight) to do that. Note that if your setup is such that the arm will project the iMac farther away from the desk, that will increase the torque still further -- if the arm extends out an additional 18", then that's double the torque.
This is a simplified explanation, since there may be lots of different objects at different points on your desk exerting different amounts of torque, but you can essentially think of the whole thing as a teeter-totter or see-saw, with the iMac as a small child at one end. You want to set things up so that there's the equivalent of a large adult at the other end, and there's no possible way that the weight of the iMac could even begin to cause its end to move down.
