I think much depends on the quality of the flannel. The lesser grade flannels tend to shift and stretch more than higher quality flannels. But flannel in general will stretch more than the cotton we typically use in quilts. If you've not pre-washed your flannel, I would suggest using a smaller stitch length as well (unless you anticipate lots of un-sewing!). Otherwise, I've continued to use the traditional 1/4" seam with not other issues.