It depends on your setup and what you are trying to accomplish. Personally I try to avoid putting anything into the common/lib of tomcat. I put all the jars required for an app in it's own lib directory. Yes I have a lot of duplication of jar files, but I can jump from version to version on an app specific basis. In my case space isn't an issue and I don't have to worry about breaking something by upgrading an app to a new version of a jar.