As I go through my planning process I have in the back of my mind which projector I'd like to go with, but that may change 3-4 months from now as I get closer to installing one. What's the best way to allow for the possibility that the PJ may need to be mounted closer or further than I'm currently planning? Do you just leave a lot of extra slack in the wiring? Multiple conduits? What about power?
Put a run of Romex up in the ceiling with a lot of slack and directly over all the possible locations. When you know what you are doing, cut a hole for an old work electrical box, reach up and grab the romex and do your thing.
You can run a conduit with multiple T connections along the possible locations up in the ceiling, Again when you know what you want, cut an old work box size hole. reach up and fish your HDMI to the nearest T.
Of course knowing your final locations in advance would allow you to do these ceiling cuts with soundproofing in mind.