Bungee cords (or those rubber cords with hooks on the end) may seem to be a safe way to secure a TV, but there are some other things to think about. I know of someone who lost an eye to a bungee cord, so I am very wary of them.
1) A bungee cord is basically a large rubber band or piece of elastic. If you've ever had a rubber band snap when you are holding something it is attached to, you can imagine what might happen if the elastic in the bungee cord snaps.
2) The information I've seen about bungee chords says not stretch them too tightly because it puts more stress on them. if you don't pull them tight, you can't keep a TV from moving around.
3) If you stretch the cord too tight and it gets away from you, the cord breaks, or the hook breaks or the attachment of the hook to the cord breaks, one or the hook ends will go flying. That's how the person I know of lost his eyesight. If you don't believe me,
here's a link to a page about the danger of bungee cords to the eyes and here's
another one
4) In an accident, elementary physics says that things that are not secured will keep going forward with a force of their weight times the speed you were going. So if the TV weighs 10 pounds and you are going 30 miles per hour when an accident occurs (or even a sudden stop), the TV will want to keep going at a force of 300 pounds. Most of the bungee cords I have seen are only rated for between 10 and 20 pounds. Enough to keep the TV still when you are moving, but not if you have to stop suddenly. And, if your car turns over, the TV and milk crate or whatever it was on that stayed secure on the floor before the accident, will be flying to the ceiling.
There are safer ways to keep things secure. You can get cargo tiedown straps at places like Target and Home Depot that can be tightened down securely and won't stretch in an accident. Other people have also mentioned the bags that are meant for holding TVs in the car while you are traveling. Those are safe ways.