I've built models as props several times for movies/tv shows. Usually its stuff for the set, but at times it has been a model featured in a scene. For set decoration you have to build things well enough that other people can move it around and not have it fall apart (sometimes a little difficult for fragile designs). There is usually an 'on set dresser' in charge of moving things on the set when shooting is going on; a good dresser will be mindful of the delicacy of things like models, glass items, etc.
If its something that an actor is going to handle in a scene, then you REALLY have to make it sturdy and usually make more than one. I had to make models for a movie once that the actor was going to handle; she was to crush one model with the other one. So one model had to be sturdy and the other had to be fragile enough to break away. It was a little stressful being on the set that day hoping that everything would work as needed.
Mark
