It is traditional to use a wider seam on flannels because they unravel more than cotton does. that would mean you would need to add 1/2inch to all measurements on the pieces. I made a quilt with flannel and just did 1/4 in seams, it seemed to be ok, they were all heavily woven, not loose. The choice is yours, but I might go with the thicker seam, due to it being a dog blanket and most likely getting washed more often than a bed quilt or throw.