We've typically just run our wire (structure, typically a single coax with 3 cat5e) to a single gang right next to the electrician's power. It's usually just on the opposite side of the stud that the electrician is running power to. We don't worry about the mount until later because a lot can change over the course of the building of the house, including both the technology available and the whims of the homeowner. We've not had a problem finding a suitable mount for the tv that the client ends up going with. If we ordered the tv immediately, they could easily change their mind and we'd wind up sitting on it and possibly not being able to use it or sell it.