Excellent question. There are many ways to design a conference room, depending on requirements, budget, etc. It might be possible to put enough pre-wire in to account for everything, but it's not nearly as efficient as designing it first.

Also, if the room will have a drop ceiling and framed walls (assuming normal commercial construction with metal studs), with the exception of AC power to the projector(s), displays(s), and screen(s), it's possible (and common) to do all the wiring after the room is built.

Joe Gordon