I think it depends. Yes, they do tend to get trailers out ahead of finishing the movies (hence why some trailers include cut scenes), but if a film is intended to be 3D from the get-go, then they use cameras and techniques intended for that. So, there might be minor tweakin, but probably not major tweaking. However, the choppiness of trailers can often detract from the cinematography and 3D can be affected by the projector a theater uses.