Is it safe to install a heating duct register in the ceiling directly above a home entertainment center?
As HerrBag stated, heat and electronics don't get along well. Remember however, Your electronics will often run warmer than what your heating the house at and all electronics have a recommended temperature range for operating. I would encourage you to review the manual that came with your electronics.

For instance, the recommended operating temperature of my lcd tv is anywhere from 41 to 104 degrees Fahrenheit. In the summer we run the AC, in the winter we heat the house to 70. Now the TV itself will give off some heat, but not enough to bring moving 70 degree air up above 104 degrees. 

I find that humidity is more of danger to electronics than heat or cold, but again read your manual.