One other thought. A layer of 30# felt will prevent nailheads in the roof boards from contacting the underside of the metal roofing, and forestall any damage or corrosion to the paint coating.

Personally I'm comfortable with rough sawn I"x boards on about a 36" purlin spacing, for metal roofing here in NH. Especially if the roof has 1x4 strapped horizontally for screwing the metal to, and ventilation. I always want a water shield (felt etc) underneath due to condensation etc. Closer purlin spacing is going to look awfully 'busy" or heavy, aesthetically.