NPHard. Funny.
My first question is, why are you bothering to save 4 bytes here and there? It's not like we're running out of space on our floppy drives?
It's useful if you need to type the private key into an import dialog. For example you may keep your private keys in a paper wallet.
Hm, in this case the overall length isn't as important as having visual breaks. Think about the way Microsoft prints its product keys that people have to type in to activate Windows.