The trailer should be your last resort: First decide if you want to see it or not.
Many moviegoers like to know as much about a movie as possible before they see it. I personally love to go into a movie without knowing anything about the plot or characters. Trailers often show the best parts, especially if the movie is a comedy. One of my rules is: The worse the comedy is going to be, the more funny parts the trailer shows. A comedy is the one type of movie I would recommend never seeing the trailer for.
An example of a comedy trailer that shows too much is this one for Bride Wars, starring Anne Hathaway and Kate Hudson. This shows one bride's actual wedding and the confronation there, as well as many of the pranks featured in the movie.
Some trailers for dramas can also show almost the entire movie. A trailer that, I think, gives away too much of the plot of the movie is the one for A History of Violence, starring Maria Bello and Viggo Mortensen. After seeing the movie, I realized it would be better without knowing anything the trailer gave away.
There are benefits to catching the trailer. They are good for seeing the tone, feel and look of the movie. Most trailers also provide the basic plot setting and a few glimpses of the contents.
Still, I would use the trailer as a last resort. If you go through the rest of the steps on the site and still feel the need to see a trailer before you go into the movie, go ahead. But it is very enjoyable to see a whole movie without knowing anything about it or where it is headed. I even go late to the theaters so I miss the trailers. for other movies.
Here are two trailers for movies in a similar genre. They are both dark action movies based on a franchise.
This is an example of a trailer for a good big-budget movie (94 percent approval on RottenTomatoes). Having seen this movie, I think this trailer shows too much.
This is an example of a trailer for a movie that did not fare so well (18 percent approval rating on RottenTomatoes). I do not know if this trailer shows too much because I would not see this movie in theatres, where it currently resides.
Both feature explosions, violence, creepy music, unsettling villains and some dialogue to frame the story, but I do not know if it is obvious to the average moviegoer which is the better movie from the trailer. This goes to show that anything you can get from the trailer, you can get by looking at who made the movie, the genre, and the reviews.