Conduit is not at all necessary if you have a straight shot between the TV location and the equipment location. If there is some barrier (wall stud) you need to get around, then definitely put in at least 1.25" conduit. Do NOT run your cables in this! Leave the conduit open for future cabling.
Make sure you pull your cable/antenna feed where you want the cable box to be. This typically is NOT to the TV location. But, if you need it there, then perhaps pull two feeds.
Run HDMI to the TV (at least one piece, but several if you aren't using an A/V receiver. Also, run cat6 cable as you will want network at the TV as well as at the A/V equipment location. Consider any video gaming that may need cables (Wii/X360Kinect) and plan for a way to get those in place as well.
Directly below the TV for equipment makes it super easy to run cables later. But, if you intend to put surround sound in place, you need to run the right cables and you need to do it before drywall goes up.