No... It's random. All magic items you can get either 1 or 2 stockets (depend on your luck, Normal, nightmare, or hell game don't matter ). All unique and rare items get 1 stocket.

But there is a rumor I hear, when you can always get 2 stockets to a magic items is, make a new char, after you power him up to act 5, stocket the item just after he finish the quest.