If you're building a Mage, you absolutely must level up their Warfare skill! It's a golden rule!
Not for me. I've only started listening to this advice recently, but I almost never put it into practice. My Mage survives just fine without it. Although, I won't deny that sometimes I protect him like the apple of my eye. But what can I do if it's more important for me to level up extra Magic skills than Resistance and Combat?