Tom, I have a two car garage and move my WSM in there for the winter whenever I use it I place right at the opening of the garage and leave the door wide open the entire smoke. I do light my chimney outside down the driveway. I also crack the side access door, however, I don't know if that does any good.
Of course use common sense, move the gas cans away, if you have any, and any combustible materials. I'm going on four years with my WSM and not had a problem cooking in the garage.