If so dense it feels like cardboard, that doesn't for comfort make, in my opinion. Too far apart and you risk lumpiness and the whole thing falling apart after many washings. It seems that a "minimum" of 10" is much too liberal. That sounds more like an absolute maximum, but even so, makes me shake my head. I'm of the mindset that 5" is a better maximum (you still get a fluffy quilt) but that some motifs have lines as close as 1/2" apart in places, which is very aesthetically appealing if it enhances the piecing.
Probably a happy medium for making sure that 12" blocks won't bunch up would be to at least stitch an X through each one and of course also "in the ditch", right?