Yum uses RPM for most of its handy-work. In the end, it doesnt matter if you install an rpm by hand or with the yum tool. Yum just makes things quicker and easier. People usually dont get into real "dep hell" with RPM unless they "use
the --force, Luke."
And the --nodeps ! So in fact, it's more safe because when you install an RPM by hand, "one day", you'll do a --nodeps because of X or Y obscur reasons, and "one day" the distribution will be broken ( i don't mean it's not reversible).
in yum 2.1.X releases you can do:
yum install /some/file/on/disk/package.rpm
it will install that rpm and resolve dependencies from the available repositiories
CentOS is still using yum 2.0.8 though. The duration of which is unknown.