This may be downvoted, as I may be way off base (and if so, please do!), but...
My understanding of the metal corners is that corners get bumped. A pure plaster corner would obviously be smashed once something runs into it. Hence, the metal bead corners.
The bead on the far outside of the corner is ever-so-slightly rounded and raised compared to the 'flange' of the corner, which is then embedded in mud. This raised edge is basically the 'form' for the top coats of mud. As the knife will 'ride' the bead, it will always be ever-so-slightly exposed on the outside-most edge.
And then, in the end, it's covered in paint.
Some have suggested that there should be a thin layer of mud completely covering the outside of the corner. My concern with that is:
- it's hard to do since knife would be 'floating' above the actual bead
- the thin layer of mud would be much more likely to damage if bumped and...
- as such, a lot harder to patch if it is bumped
In summary: I've always left the outside-most edge of the corner bead exposed and then paint over it. I've never had an issue with that myself (but I'm also not a professional sheetrocker...)