[CentOS] when is the o.s. considered to be at a certain minor version? Or, is it safe to apply only certain package updates from the next release version?

Two related questions about the minor release numbers (e.g. the 'x' in 5.x or 6.x) :

1) What constitutes the o.s. being at a particluar minor release?  Typically, when you install you are getting a package set available from a specific minor release number.  But what minor release is the o.s. at if I just update the centos-release package, and no other package?  Typically, a 'yum update' is said to take your whole package set to the latest minor release.  But what minor release is the o.s. if you just update certain packages (instead of taking all avaailable updates)?

2) Can I apply package updates made in a minor release greater than my current release without detriment to the integrity of the o.s.?
E.g. Suppose all my packages are at versions delivered in v5.6.
Suppose also, that I have a package xyz installed, and that an update to it was made available in v5.8.
Can I upgrade xyz to the updated version from 5.8 without updating other packages (except for any dependencies xyz has) to the versions available in v5.8?

Thanks,
-- 
Jon Detert
Sr. Systems Administrator
Infinity Healthcare
Milwaukee, Wisconsin
414-290-6759