I would not X it unless you prefer that look. I have only done one rag quilt and all 3 layers were flannel. The largest squares were (I think) 9" cut, 8" finished, and I didn't sew and X and it's fine. Here's a link to my quilt to see what it looked like without the X.
http://www.quiltingboard.com/picture...t-t216760.html I will say that with 3 layers of flannel it is pretty heavy.