I haven't really been paying close attention lately. I do know I got a 60% off from Joanns just a few weeks ago, which I think was pretty rare. I got a whole bolt (12 yards) of 108" backing fabric for under $50. I am SET for quilt backs now, as long as solid navy blue works with the quilt!
It has seemed like 40% off coupons are a lot more common than the 50% off these days. Honestly I usually use them at Beverly's anyway....they only honor up to 30% off on fabric on competitor coupons and only one coupon per transaction and they won't take online/app coupons, but they have LQS-quality fabrics there that Joann's just doesn't have. So when I have Joann's flyer coupons and I am going to use them for fabric, that's usually where I take 'em!