No, they show you the best scenes to make the movie look better than it is. They’re crafted not to give away major plot secrets, though. Okay, you can see somebody shoots somebody in the trailer, but by the time you see the movie you don’t know who shoots whom — if you ever did.
A lot of trailers I’ve seen lately give the whole story. But they have gotten away from juxtaposing two scenes that have nothing to do with each other and making them seem as though a lead character got blown up during the movie.