Our new house has a section of roof with exposed rafters to be covered by 2" T&G cedar planking. Over this will be placed sheets of 2" rigid foam insulation. The roof will be sheathed with plywood and the roofing to be nailed on to this is an artificial slate product. Question: Does the insulation need to be covered with 2"X2" strapping to provide an air space between the insulation and the plywood roof sheathing or can the roof deck be laid directly on top of the foam sheets?

Probably not, but the question really is, even in your moderate climate, is 2 inches of insulation enough? That's in the R-10 to R-12 category, and I don't think the cedar decking will make up the difference between that and a minimum of R-19.

A gap between the insulation and shingles is to cool tar based shingles. If the shingles are not damaged by heat it should be ok.