You don't "have" to do anything, but the optimal position will depend on a number of factors (size and power output of the satellite speakers, location of the display relative to the room, flooring type, room shape, etc.).
At the most basic, the center channel speaker goes directly above or below (I'd recommend below if at all possible) the display, aimed directly at the primary seating area. The subwoofer is non-directional, so it doesn't matter where you put it, but corners are best.
Satellites traditionally should be aligned with the top half of the display, so if the top edge of your iMac is 45" off the ground, none of your speakers should be higher than that. This isn't always possible, so generally just ensure that the left-to-right height matches. The front pair could be higher or lower than the back pair if the room requires it. You'll also want to adjust the output of the individual speakers so that the sound is balanced (same volume from each of the four satellites and an appropriate center channel volume based on your preferences). You can do this with most quality receivers.