There are 2 things to consider when deciding how closely to quilt.
1) The batting can migrate causing the batting to bunch up inside the quilt & get lumpy and uneven. The recommendation on the batting package for the minimum amount of quilting will prevent that.
2) The fabric and thread can become weak causing snapped stitches, frayed thread or fabric, pulled threads, holes in the fabric or thinning/worn out fabric. The way in which the quilt will be used & laundered and the intended life of the quilt will determine the minimum amount of quilting to prevent those issues.
In general, with cotton fabric (and even cotton-poly blends), I wouldn't recommend quilting farther apart than every 4" unless it is a "throwaway" quilt that you only plan to have for a year or two. Quilting every 8" for anything that you plan to wash more than once or twice a year and that is going to be used (as opposed to being hung on the wall) is bound to start developing issue sooner rather than later. I've done quilts with that little quilting & consistently had issues with them. After fixing a few, I started researching more & found that for quilts that will be laundered on a weekly or bi-weekly basis, quilting really should not be more than 2" apart. For quilts that are going to be used on a bed or as a lap quilt and will be laundered every couple months, a minimum of quilting every 4" keeps me from having to re-do the quilting every other year. Doesn't have to be fancy. Just divide the rectangles into thirds & quilt straight down the quilt.
