No, that's not necessary. It's sufficient to reload the web application whose web.xml file you have changed.
Joined: Aug 24, 2007
Is there any event after which we must start tomcat? or reloading helps every time
Joined: Mar 22, 2005
If you make changes to the Tomcat configuration (as opposed to changes to one of its web app's configuations) you need to restart the server. Any of the files in TOMCAT_HOME/conf would be examples of this.