I always delete my zip file after I have extracted the files inside. I thought pkgdelete was suppose to delete every file that is associated with the package. I see see files in /usr/local/man or /usr/local/share that are part of the package that I deleted. I've used pkgdelete to delete a package and it removes the binaries and libraries asociated with the package, but I notice it doesn't remove everything. You could move them to a external hard disk or burn them to a CD or DVD. I would keep them just in case you need to reinstall those same applications in the future.
When you’re done installing the app, you can delete the PKG file and any DMG file it came in (after ejecting it, of course). These sorts of installers can do things the drag and drop installers can’t do, like installing system services and putting files elsewhere on the computer.