"Sidenote topic: In the SMD Nightwing Tarp instructions they say "The use of binding tape on the front and rear edge seams and the ridgeline is very important. These seams take the bulk of the stress of the shelter and minimize the stress on the canopy panels. Reducing the stress on the canopy lenghens its life."
On a two piece tarp, I would simply roll the seam an extra time. This is the same as a French flat seam but stitched three times, rolled twice. This will take up any extra stresses with little weight addition. Along the edges, just roll the cut ends in and stitch twice...same principal. Generall, a flat canopy WILL streatch a bit. So, it is a bit easier to compensate for it during construction, say a slight cat cut, than just letting it go flat. But, doing both is a big, overkill. My tarps generally have streched about 1/2" over about 5 years of use, at 30-40 nights per year. They needed a recoating to repair the misting, otherwise they were still servicable.