Stitch-in-a-ditch spacing

I would like opinions. When I quilt, I stich-in-a-ditch along the horizonal and verticle lines between the blocks (I don't know how to free-quilt). My blocks are usually 12 inches in finished size, though I do the same for smaller blocks.

Someone told me that her rule if thumb is spacing no wider than the width of a fist, though she knows that most battings say no wider than 12 inches.

So far, all mine are and probably will be s-i-a-d, except for a full size for my niece which I'm having long-armed. All mine are meant for use, not heirloom.

Is 12 inches too far apart? What do you do and how do you decide where to quilt ON the block?