It depends-
The answer depends a lot on how well you think those straps can withstand the forces in a crash. If it is a picutre tube TV rather then an LCD screen, then it could easily have 500 pounds of force or more in a severe frontal crash. If it does break loose, it could seriously injure an occupant.
I have a bag VCR with a small LCD screen. I mounted it with straps behind one of the front seats, wedged into the space in front of my daughter's rear-facing carseat. I then attached the lightweight LCD screens to the headrests. You can see it here:
http://www.pbase.com/image/16936168.jpg
I think the risk of the bag section becoming a projectile is much lower this way, though it would be even better to put it under a seat. The LCD screens won't become a projectile, but they are still a small risk that the child's head could hit them in a severe crash.
For these reasons, we only use the video system on long trips. I don't have it in the vehicle for day-to-day use. There is certainly added risk depending on how you install it, but only you can decide how much risk you will accept...