We’re really talking about two different issues.
1. Is it bad for your TV to be above the fireplace?
There are two things that are bad for your TV: heat and debris. Your fireplace creates both of those, but not to the degree most of us think. If you’re like most people in Southern California, you MIGHT use your gas fireplace once a year. In that case, it’s probably not going to be a problem at all. Gas fireplaces don’t release a lot of smoke and don’t create a lot of heat.
If you want to be safe, put a thermometer above your fireplace and light a fire. Get it really hot and see how much the temperature changes. Check the safe operating temperatures of the TV to make sure you’re still within the recommended levels.
I’m not going to say that installing a TV above the fireplace is totally safe because as soon as I do, I’m going to get sued for someone’s house exploding. I will say that I’ve installed a lot of TVs above the fireplace, and I’ve never seen any problems arise from it.
2. Is it safe to run wires around your fireplace?
Improper installation of cables around a fireplace is a fire hazard. The fireplace should have at least 2″ of clear space around it. That means extra care should be taken when installing wires around a fireplace or smoke stack.
All wires should be secured so they cannot come within 2″ of any part of the fireplace or smoke stack. That usually means you have to open the drywall to install straps.
Be wary of anyone that DOESN’T open up the drywall for a fireplace installation. They are probably cutting a corner to save them time and money. You shouldn’t take risks when dealing with fire.