The panel mass of a plasma unit is significantly higher than a LED-LCD, which makes them (plasma) prone to transport damage - if not secured properly. The packaging for panel televisions is engineered to optimally support the load distribution of the unit inside, in a vertical orientation. That's not to say you can't transport one in a car, truck or SUV laying flat - just be aware of your roadway. The mass/weight of LCD units is so much lighter it's really not an issue, even more so now that LED based units have lightened things up even further.
It if makes you feel any better you can always open the box before you load it, to make sure to orientate the box in the vehicle with the screen surface facing upward.