This could depend on the language and/or environment you have, but for projects I work on I place no libraries (jar files) in source control. It helps to be using a tool such as Maven which fetches the necessary libraries for you. (Each project maintains a list of required jars, Maven automatically fetches them from a common repository - http://repo1.maven.org/maven2/)
That being said, if you're not using Maven or some other means of managing and automatically fetching the necessary libraries, by all means check them into your version control system. When in doubt, be practical about it.
