At one time most stocks were advertised as quarter sawn. Due to what I think is popularity and such both types are now used. As to shrinkage, a good stok blank should be shrunk when you get it as they advertise them dried to 6-8%, which is about as dry as they can get them. I can only agree with Mike on the fact that there are other concerns more important in picking a stock than worrying about whether it is quater sawn or plain sawn. Some like the figure of plain sawed blanks better. The primary concern on warpage in a plain sawn plank is that it warps away from the center. After finishing a gun stock is far from a plank.
DP