My update method on two critical systems has been to use yumex.
I select all, unselect major apps that I recognize (bind, openssh, etc), update and reboot.
The yumex to get those apps and another reboot (most likely not needed).
I have updated my notebook. The only detected problems are: madwifi drivers and lost Thunderbird icons in drawers (just had to go through the properties and reselect the icon). Too soon to tell about anything deeper....
One server so far. This is a new box that I built only a couple weeks ago. Update went fine. I am now taring up the system as a checkpoint before proceeding.
Two Trixboxes. I am having that classic Trixbox problem with Centos and Zaptel, so I have to reread the docs, as yum update fails with unresolved dependencies.
So after some fear and doubt, I have made the leap!