Ronsonic 

I've built bikes both with and without the face and ream job and one before and after. It can definitely make it easier to get the cups fully and squarely seated. I'd say rarely necessary but often beneficial it only needs to be done once, it's just good practice.
Unless something makes it impractical, just do it.
Supports and helps build a relationship with you local shop for not a lot of money.

I'm even more strongly in favor of chase and face for the bottom bracket. That's another topic.