In a standard film scenario I think I would fall on the side of the cinematic approach, where panning corresponds to the direction of the helicopter as opposed to what would happen in reality. I guess it's a similar argument to the way distant explosions/lightning strikes are traditionally treated in films; you hear them as the event is seen rather than after what would be a small delay in real life. This is one of those crazy conventions that Fred and Roger refer to.
Incidentally I was reading some comments on a Battlefield 3 trailer earlier and everyone was complaining about hearing the gunshot at the wrong time i.e. it wasn't realistic. I think people were more intent on pointing out the fact that they knew the gunshot would come later rather than trying to improve gameplay - I'm sure the sound designers at DICE are well aware of the physics but chose this approach because in such a fast paced game it would be utter chaos (not saying that you have the same reasoning for bringing this up ofa :D )
I also agree with Shaun however that the sound treatment is relative to whatever is happening in the story at that particular moment and/or the style of cinematography. Maybe it would benefit the scene to intentionally make it disorientating and flip all the panning round (I quite like the idea of doing that for something....dibs!)
So anyways, back to the original scenario, maybe a good way to illustrate the building reflections would be to pan the helicopter in the "correct" direction, and send the same signal to a bus panned in the opposite direction with 100% wet slapback reverb of some kind? So you get the dry signal coming from the helicopter on the right and a slightly delayed reflection bouncing off the building on the left sort of thing.