1. why use a beta; makes no sense.
2. its very unlikely anything working on 5.8 will not work on 5.9; its just minor updates/bugfixes.
Unlike going to the latest major version 6.x series.
When someone says their SW will 'only' run on eg 5.8, what they usually mean is 5.8 minimum.
Just try it on 5.9.
If you are using RHEL, just disable the rhnsd daemon via chkconfig cmd http://linux.die.net/man/8/chkconfig
You can then manually control whether or not to check for/upload updates.
I'd recommend keeping it up to date for bugfixes and also security(!) reasons.