John, the two major differences between flat sawn and quarter sawn are in shrinkage and resistance to weathering. In eastern white pine radial shrinkage is 2.1% and tangential is 6.1%, so in service quarter sawn will produce much less splitting movement against fasteners. If shingles are green when layed and blunt point nails are used, you should not experience much splitting. Quarter sawn or vertical grain shingles are more resistant to erosion. A copper ridge cap and flashing will inhibit fungi. For maximum life of roof, paint the shingles. Historically, the wood shingle nail base was more like 1 x 3 rather than 1 x 6, in this case more is not likely better. Shingles require ventilation and air exposure on the underside.

Also the Forest Products Lab web site has many resouces for free download. http://www.fpl.fs.fed.us/