Bonjour
If I understood well the Arnes "bolt rope" concept the rope or webbing should be stiffer than the sailcloth, in order to transfer the load from the sail cloth to the corners of the pannel and the sail. Otherwise the "bolt rope" or webbing would not take the effort. If the webbing is sufficiently stiff it will be much stiffer than the stitching were the thread is already going up and down through the two or three material layers...
You may, mainly for intellectual matter, specially because it is at no cost, add a little extra streaching by using zigzag or half zigzag to the stitching.
Anyway the relatve streatching of the sail cloth and the webbing is much more important.
Eric