Do I need "Install OS X Mavericks" after I've upgraded?
No. If you have no use for it, delete it. The OS doesn't need it any more and if you want to reinstall OS X you can use the Recovery HD, so there's generally no need to keep it.

The post-install script will even delete it for you if its found in the default location as far as I know, but if you still have it there's no problem deleting it yourself.