I think that the 10" means that from one quilting stitch there should be another quilting stitch - in all directions. That would mean that you should cross-hatch your quilting lines at least every 10". A more extreme example would be if you quilted lines that were 10" apart. You would obviously need to crosshatch them with another set of lines 10" apart in order to meet the rule. What if they were 8" apart? Same answer, probably. But as your parallel lines get closer, this rule seems to fall apart logically, and if you think about tied quilts, the "stitching" would never be close enough. So, my answer is, I don't really know, but I think you will be fine.