Typically when I'm ready to release, and after I've done every other change, I will bump the revision in my AndroidManifest.xml and generate a new .apk from eclipse, then test it out on my device. If everything looks good, I'll commit the manifest change, along with the now updated proguard directory, with a commit message containing the release number. Then I'll tag that commit with the same name. This way, if that release ever has any exceptions I need to debug, I can download the obfuscated stack trace from the Android developer console, run git checkout <TAGNAME>, and use the proguard files from that release to deobfuscate the strack trace.
Keeping the proguard files under git makes them quickly and easily accessible, should you ever require them.