Magic penetration item lategame

Has anyone done the math on building magic pen item late game even when enemy team isn't building magic resist?

For instance my usuall build is Blackfire torch, Lichbane, Rabbadons and Mejai. I always build cdr boots.

So for the fifth item slot ill go Banishes some games when i get hard cc often or Rylais if i feel like i need to kite and my team has enough dmg already.

What about games when i want just raw damage?

Is it better to get Void staff or the other magic pen item late game when everybody is high lvl already because of the basic magic resistance even tho they are not building magic resist, or am i better of with Shadowflame, etc.?