It doesn't matter more for the off-hand than for the main-hand. You will still use the highest median damage weapon in main hand. Median damage is (Max+Min)/2, if you use median damage you can ignore speed because quicker weapons will automatically have a lower median damage.
And since you are dual wielding you will have a penality to hit (and damage) to your off hand. So your priority should be your main hand because of special abilities (opposed to auto attacks). As for the off-hand you will want slow weapons (and high median damage) because unlike rogue who will want to apply quickly poisons you will want to do more damage with each hit.
Higher median damage is always better for a death knight.