By looking at the man page, distro-sync wouldn't re-install a package with the same version. For exemple, on my rhel boxes, I have tzdata-2011g-1.el6.noarch installed, and on my sl6's one, I have tzdata-2011g-1.el6.noarch. It's the same exact version number. So the installed package wouldn't be re-installed.
You would need to do yum reinstall * to properly re-install the package from the new repo (sl6 or c6 for instance).
Are there many packages with identical names that you'd *care* about reinstalling? tzdata for example I assuming is functionally identical (if not pretty much bit-for-bit). Given it's got the same NEVRA, it's not going to get updated any differently either.
tzdata was just an exemple, it was the first one I tought that was updated frequently...
For my part, no I didn't care and left those packages that were already installed alone.