I have done some trailers for films and even more often have mixed trailers for theatrical release to promote television shows as well as television tie-ins using partially completed stems from feature films to make promos for behind the scene footage and sneak peaks,etc. I almost always get stems from the house that is mixing the actual film/programming.
I try to be as to true to the original as possible but as Glenn mentions above there will be times where there must be provisions made for "the sell." I have juiced up or thinned sounds to "up" the excitement of the cut. Often times the sounds that make the furrows and peaks that are necessary in the success of a long form story line must be tweaked a bit for a trailer or promo in order to make the specific cut flow and crest and grab the excitement of the viewer. It is a hard balance between wanting to design and mix something that will "sell" the audience but remain a true representation of the programming, especially as often the actual programming is not available in its entirety for me to view. I find you have to ask yourself if what you are adding is necessary to the successful communication of the message in the few minutes, 60, 30, 15, or 10 sec you have and if it still represents what could reasonably be within the sound palette of the actual programming.