Also since the bridles are tied to the cross spars the spars would bear the pressure on the spine as pressure against them rather then pulling on the ties at the crosses.
I build my larger roks with a 6 point bridle i.e. bridle line to the spine/spreader cross points as well as the usual spreader points. This removes the need for the "shoelace" ties at the cross point, as the bridle loops around the cross and hold it in place.
I've pondered the "over or under" question for my setup a number of times. Tried both ways, they both work fine.