Offline backups from App-store downloads


When purchasing/downloading OSX apps from the App store, is it possible to keep an offline backup as well?
I've been told that you can simply copy the app itself from the /Applications/ folder, but of course that only assumes that the installer didn't spread lots of additional support files all over the hard drive.
The assumption is largely correct. App Store-compliant applications are generally self-contained. That said, local backup of App Store purchases is unnecessary because you may download any purchased application to any Mac on which you have signed into your App Store account.