Yes, W296 requires a magnum primer, but someone will post that they use standard primers rerularily. Large charges of slower burning ball powder are best ignited with a magnum primer.

I don't use W231 'cause it's too fast, IMO, for the 357 Mag in anything other than light target loads. I know you asked about W231 and W296, but ther are other well suited powders out there. I use a lot of 2400, True Blue, and Unique for my .357 Magnums.