This depends largely on your goals -- and how complex a system you are running to begin with. Yeah you don't need to know exactly how Kratos the indomitable got a +10 to attack, but if you say he is a level 5 fighter, you should have an idea of how he could get there.
But why should you even know he's supposedly a level five fighter? What in a non-metagame sense would tell you that?
@Lanefan has made the point repeatedly that if for some reason you need to hand control of an NPC to a character, that character should look like a PC in terms of how they were built. I agree that this is the ideal situation, and this should be something you can do without a lot of hassle.
But of course in games that do that, you almost never expect that to be necessary.