Below are the possible scenarios for EL evaluation by container....
1. Not in
JSP and web.xml:
By default in a JSP and web.xml, if you do not specify the isELIgnored attribute for page directive and <el-ignored> tag respectively, it will be considered as isELIgnored=false i.e. the EL tags are evaluated in JSP
2. Specified in web.xml and not specified in JSP:
In this case the value specified for <el-ignored> is considered.
3. Specified in JSP and Web.xml or Specified in JSP and not in web.xml:
In either case the value for isELIgnored attribute for page directive will be considered when the page is translated by the container.
To answer your question
1.whether we need to use isELIgnored only to override the default behavior of what is specified in web.xml.....
Ans: Yes, but provided you need to verify the <url-pattern> also.
2. To be used in conjunction with web.xml
Ans: Not required.