If you're making a pure cold sorc, you should put some points into cold mastery, but maxing it isnt always necessary.
Remember that all non-immune to cold enemies have a max cold resist (like Duriel in hell) of 95%, and remember also that in hell mode resists cant be lowered over -100%.
So, your resist-lowering has to reach a total of -195%, including both cold mastery and gear bonuses (facets, for example).
You have to consider your +coldskills in your gear, too. If you use 3 perfect cold facets (common solution in a pvm sorc with a facet in shield, nightwings and fathom), you need a -180% to have the max effectiveness of your cold mastery skill, so you need to raise it to lvl 33 (including gear bonuses). This means that you dont need to max it if you have at least a +14 (or more) to cold skills by your sorc's gear.