createrepo question

[Aldo Foot]

Right now I'm running F8, kernel 2.6.23.8-63.fc8.

I've setup a local yum repository, which gets updated via a cron job.
The log files show a number of newly downloaded updates.
No updates were showing using only my local repo until I ran
createrepo against the dir containing the base, updates and repodata
directories.
So, after a few days of no seeing anything now my system picked up 82
new updates. That was good.

The question is:
Do I need to run createrepo every time new updates are downloaded?
Does createrepo need to update the metadata file for checksums and such?

~af