Actually, I believe you need to halve the distance for knotting, especially if the batting is polyester. If the package says quilting lines can be 6" apart, I would plan on knots no further than 3" apart from each other. My experience with poly batting is that it will pull apart with repeated washing if the knots are not close enough. Knots provide less structure for the batting than quilting lines.
I haven't done a knotted quilt with cotton, but I would think that a needlepunched cotton would be more stable. I am thinking specifically of Quilter's Dream 100% cotton, or Mountain Mist Rose (which is available in my JoAnn's). With those battings I would be more comfortable making the knots farther apart. However, you would not get the loft of, say, a Hobbs PolyDown.