The uninstaller will remove Growl from both of the places where it might be installed, skipping any where it isn't.
Some applications install Growl without your permission. We hate this, too—no program should ever install any other software without your permission.
If you use Dropbox, you must also turn off its Growl support in its Preferences window, or Dropbox will reinstall Growl the next time you start your Mac. See our page about applications that install Growl without your permission for more information.
If you are uninstalling Growl for a reason that we might want to know about, please contact us through one of the methods on the contact page, and we can look into fixing those problems.