You're right about facing, though I understood it to be more about removing paint and correcting manufacturing tolerances to ensure that both faces are in fact parallel. The issue is that when the cups in the head tube or bottom bracket aren't parallel, the bearings are unnecessarily stressed and will wear unevenly.

The good news is that your LBS is likely to be able to do it, and can tell you if it's necessary on your frame! It's a relatively quick job with the right tool, so it's not usually expensive, especially if they don't need to remove the headset races and BB first.

So it's not too expensive to contract out, and it can extend the life of any bearings you're using in the bike. I've only ever rebuilt frames, so I just stick with the headset and the BB face is usually paint free already.