People seem to have got good sail shape with a range of yard stiffnesses.
Richard Harvey has a wooden yard that I think bends about 50 mm or more with 10 kg on it (anyway, standard as far as the plans go as far as I know), and very good sail shape from McNamara sails, though they had to re-cut for greater head edge round after seeing how much the yard bent.
Woodeneye went to an alu yard that bent much less (20 mm with 10 kg weight?) and that worked well with his sail.
So, bendier yard - more edge round, stiffer yard - less edge round.
How much? Stewart Dabbler works it out by hanging 'one third of the square foot area of the sail in pounds' off the middle of the yard, and measuring the deflection of the yard at that weight, using that amount of edge round along the head of the sail. 'one third of the square foot area of the sail in pounds' equals 35 pounds or 16 kg for the GIS sail.
My sailmaker agreed with this, I gave him bend data on my yard at 16 kg weight hanging off the middle, he cut the edge round based on this. He said the shape is in the broad seaming. I guess if you were mainly using edge round to establish shape in the upper part of the sail you'd have to add a bit more edge round than this.
Ian