In trying to get things in place before drywall goes up, even though I'm not going to install a projector immediately, I do want to install a wiring run in the ceiling to keep the option open in the future. Is there a standard distance to work with from the screen to the projector mount?

If I understand your question correctly....

Each projector has a min. and max. distance based on how wide your screen will be. This link will help you understand what I mean and tell you the best distances for your projector.

Screen size vs. distance calculator.

Bud I did pre-wire for a projector knowing that there could be a variable distance depending on screen size and projector type as per Murph's post above. Not knowing what projector or screen size I might use in the future I ran a couple of extra 8' studs length wise inside the ceiling prior to hanging the drywall so that a projector bracket could be connected anywhere along this 8' length for the distance I'll need at the time, I also made sure to run enough cable inside of the ceiling to cover the length of this 8' distance.
In my particular room I calculated the 8' *track* distance would have been sufficient back and forth movement to cover just about any screen/projector combination I'll probably ever use. I also utilized a blank wall plate for access to the wiring.

Great idea Rick. This is again an option that I want to have in place as a "just in case" scenario. I really have no idea what type projector I would need. I do recall reading that some projectors are "long throw" and some are "short throw" depending on installation needs. Much to be learned (and $aved) before I can consider what to get.