What ~Gnos said above.
I'd like to add,Death Runes really helpful for the attacks which require 2 diferent runes to use,[as these are always by far your highest damage and RP generating Strokes].
So for instance Frost Spec,so I use /Castsequence Icy Touch, Blood Strike, plague strike to get my diseases on to the target/slow them/and activate my Death Runes,once they are cooking I will drop Obliterate on them,or if they still need more rotations of IT/BS/PS Death runes are there for that.
Also not to forget to create a macro /cast rune tap
/cast horn of winter
This is vital if your needing to drop pestilence for AoE bursting and find your self stuck without active runes.
Also tying your HoWinter to your Rune Tap means its less easier to forget to use in combat,I find im always needing Death Runes.
The above is always for BG not Arena,hope this helps