How do *you* retrieve the release version on your systems?

I am looking for a clean/easy/simple way to gather the release version (specific only to the 5/6/7 EL releases, I suppose).
I have considered using the following files/commands:
/etc/redhat-release
/etc/system-release-cpe
lsb-release
uname -r

The problem I am running in to with any of the above methods is the lack of consistency (either due to differing build standards on my hosts, or the output is slightly different).
For example:

/etc/redhat-release (seems consistent thus far regarding the number of fields - however, Fedora is different and I have not tested CentOS)
/etc/system-release-cpe (was not implemented on RHEL 5 - AFAIK)
lsb-release (not all of my systems have lsb installed)
uname -r (parsing for el5 vs el6, etc... might be the ticket?)

I'm just wondering what other folks are doing. My bootstrap script determines which activation key to use based on the OS release and a few of my 3rd-party Repositories as well. I'm searching for the lowest common denominator(s) or most consistent way to determine the information.

Then there is things like this (notice column 5)

#  cat /etc/system-release-cpe
cpe:/o:redhat:enterprise_linux:6server:ga:server
# cat /etc/system-release-cpe
cpe:/o:redhat:enterprise_linux:7.0:ga:server

Then... notice the difference in the yum vars (6server vs 6Server)

# python -c 'import yum, pprint; yb = yum.YumBase(); pprint.pprint(yb.conf.yumvar, width=1)'
{'arch': 'amd64',
 'basearch': 'x86_64',
 'releasever': '6Server',

EDIT: I apologize - I should have mentioned that I am doing this from the client itself. So, I need a consistent command that could be run on a RHEL 5/6/7 client (Sorry for the confusion)