I dont use RHEV (yet) but being that its based on KVM and libvirt I dont think you need guest tools. When you configure in RHEV Manager does it identify the paravirtual devices as virtio? If so then its standard in the kernel used by CentOS since around 5.5. I run all my CentOS VMs on the KVM / libvirt platform and systems install fine without any additional tools. The only additional thing I install is acpid to allow the VM manager to perform shutdowns and reboots.
My bad, indeed, the guest OS works perfect. What I'm looking for is the extra data RHEVM can provide that it get's from the guest tools, like IP, memory use etc. See http://bastion.jkit.nl/~jkooijman/rhev.png. The bottom one is a Windows machine with the guest tools installed.