In general I would say pump Vit and leave Dex at base, because necro totems tend to have low block chance and I'd rather not have to worry about that when picking my shield. However, if you were planning to use a Homunculus anyway, it would be worth going for 75% block.
Of course, the type of necro you're playing also makes a difference. If you have few or no minions, blocking is much more valuable than if you have an army to stand between you and harm.
I used to think max block was the way to go for my bonemancer, but have since changed my mind. It takes 200+ to get max block with a necro (depends on shield and lvl), that's almost 400 life. Now remembering this is a bonemancer, I have almost 1k in bone armor and that generally takes care of any physical attacks. Now even with max resists (for pvm I usually don't go absorbs) you will still be taking damage, dmg that is less impactful when you have lots of life. Now I find that if I die in chaos sanctuary it's usually from magic attacking serpents, which can't (to any degree) be resisted and can only be hampered by high life.
Also as a bonemancer I usually do crowd control with prison, wall and some revives floating around. So the only dmg that generally reaches me is elemental/magical and any physical that can get through is totally taken care of with my bone armor.
As for PvP, I don't play 1.1 non ladder btw so I am speaking mostly enigmaless dueling, I find most physical dealing characters are easily contained with wall/prison/decrep and or I can take there hit and run away to recast armor. Elemental and magic dealing characters won't be stopped by my 75% block so life is obviously more important there as well.
Anyway I just find that for bonemancer especially, and I imagine skelemancer as well, life is just far more benificial than block will ever be. Though I would definetly go block for daggermancer, and possible poisonmancer.