It really boils down to matching the amp draw of the device to the amp limit of the cord. The cord will tell you how much it can carry. The cable box has a sticker on the back or bottom which tells you how much it draws. Make sure the cord carries at least that much.
My concern is not so much for me, when I use extension cords in semi-permanent installations, but for others. I know I'm not going to overload an extension cord, but I cannot guarantee the boys won't. So, my personal policy on semi-permanent extension cords (and I use a fair number of them, especially in the home theater) is to always install an extension cord rated the same as the circuit breaker. You just really won't find extension cords around my house which are not rated for at least 15 amps...no matter how long they are.
Kev