It is. But I'm not sure if there are other distros that happen to have a /etc/rc.common. Seems like the common way to do it though:
aix.rb:  confine  :operatingsystem => [ :aix ]
appdmg.rb:  confine :operatingsystem => :darwin
apple.rb:  confine :operatingsystem => :darwin
blastwave.rb:  confine :osfamily => :solaris
freebsd.rb:  confine :operatingsystem => :freebsd
hpux.rb:  confine :operatingsystem => "hp-ux"
macports.rb:  confine :operatingsystem => :darwin
msi.rb:  confine    :operatingsystem => :windows
openbsd.rb:  confine :operatingsystem => :openbsd
opkg.rb:  confine     :operatingsystem => :openwrt
pacman.rb:  confine     :operatingsystem => :archlinux
pkgdmg.rb:  confine :operatingsystem => :darwin
pkg.rb:  confine :osfamily => :solaris
pkgutil.rb:  confine :osfamily => :solaris
portage.rb:  confine :operatingsystem => :gentoo
rug.rb:  confine    :operatingsystem => [:suse, :sles]
sunfreeware.rb:  confine :osfamily => :solaris
sun.rb:  confine :osfamily => :solaris
up2date.rb:  confine    :osfamily => :redhat
windows.rb:  confine    :operatingsystem => :windows
zypper.rb:  confine    :operatingsystem => [:suse, :sles, :sled, :opensuse]

Is it not preferred to confine based on the operatingsystem fact?