I would pin it unless you have a lot of experience with roll hems. I thought I did and messed up one pretty bad. After that I pin any seam with a radius to it. The McCats have gross gain ribbon on the edges. I like them for the strength and it hides any mistakes made.
It maybe a little late for this. But my next tarp is not going to be a true hex shape. I made my current one that way. I think it leaves too much un-useable room on the ends. Plus the width was way more than I needed. I want to measure it out next time I set the hammock up, but I am thinking something like 8 feet wide and 12 feet long. Then add some fudging with the angles and the cat cuts to make it work.