When Setting Up Speakers

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.

 

I tend to find rear speakers work best slightly higher up. As matticus08 says, subs are by-enlarge pretty un-directional. If you find the bass is lacking, stick it in a corner. If you find you're getting plenty of bass, the you really should put the sub slap bang in front of you, directly underneath the centre channel.

If possible, get all the front speakers at the same height, no issues with comb-filtering then...

 

Center channel under display, or course. I like the front channels at sitting level. Then the rears I place about 7' off the ground aimed toward the center of the room to get pretty good surround no matter where I sit or stand... works for me!