Most people have something they do not want to see
If you don't have a problem with any R-rated content, you can skip this step. But most of the people I know have at least one thing they stay away from. I try to stay away from movies with torture or portrayals of violence as attractive. I do not always succeed, but I try. My old roommate feels awkward during graphic sex scenes and my friend Robert is ok with violence and sex, but hates profanity. My friend Gena cares what the movie has to say about morality, fate and humanity. My friend Alex won't see any movies with rape or statutory rape scenes. I even know a guy who won't see a movie if a dog is killed in it!
Maybe you want to avoid taking your young nephew or grandmother to a movie like Forgetting Sarah Marshall, a raunchy, but hilarious comedy. Or maybe you have standards you want to live by. A lot of people don't like to watch movies that are too dark or scary. There are ways beyond looking at the ratings system to find out what movies are not for you.
Worrying about content as an adult does not make you a wuss or a prude.
I think there are several reasons to screen movie content, even if you are an adult who is not easily offended. Many people are visual and images stay in their heads indefinitely. I would not say that all people should avoid R-rated movies or specific content, because everyone is different. You know your own weaknesses. You know what makes you a less responsible or kind person. If you are a recovering alchoholic, you would stay out of a bar. Similarly, if sex scenes tempt you to cheat, feeding that lust can only make it get bigger. If you have a tendency to be cruel to animals, I would skip the serial killer movie. I am not trying to be a wet blanket, but I think everyone has areas they need to watch that can be fed by movies, gradually infiltrating their psyches and making them just a tad darker or more cynical.
Good sites for content screening
There are several Web sites that can help you screen content, but my favorite is PluggedIn. While Plugged In is clearly a fundamentalist Christian site, it is not as strict and dogmatic as some other sites of its kind. Unless you are offended by this point of view, you will not be upset by the analysis of the messages and content of movies it provides. I actually find the reviews to be entertaining. Occasionally I even agree with the site. I recommend it because they review every movie that comes into theatres and always provide spoiler warnings in italics. It is very easy to navigate the site and archives are available to the public without subscribing. Plugged In also takes time to record the positive messages and events in movies, rather than just listing the curse words, violent acts and sexual encounters. To avoid learning things about the plot, skip PluggedIn's summary and go directly to the heading that says Positive Elements.
One of my favorite critics who provides this service (and actually reviews the movie along with it) is Nell Minow, known as the Movie Mom.. Nell Minow is a little more liberal politically, but not as biased. Although her reviews are now featured on a site about faith, her reviews are mainstream and were previously features on Yahoo Movies. She is thoughtful, thorough, and clearly intelligent.