A lot of views with no replies, seems like people don't know either :P Personally when I run into this situation I just wait for my next blood rune to come back, this is better than using icy touch because I will get 2 death runes, more SS the better hope this helps.
Easiest way would be to use a death rune and not getting into the dilemma at all, if otherwise you're sitting on a single frost rune, its still your lowest priority to spend, so just play your priority and if you get into the situation of getting no blood rune back until you can do nothing else then just IT once. You could also use a FeS on death/frost but that is probably not the best choice.