Hi!

I would say it's just personal preference. Whenever I only play a two note power chord (root and fifth) I use my index and pinky. Just feels more comfortable to me, and you get to use your pinky a little more. That sucker usually doesn't get enough practice so it's good to throw it in there in some easy stuff to strengthen it.

However, if I'm adding notes I play the root and fifth with my index and ring finger. Anyway, it's mainly personal preference.