I just pin it. I wouldn't worry about the pin holes. They are not enough to worry about. Plus mine are usually inline or close to my seam anyways. I only seam seal the ridgeline of tarps.
What works great for me in sewing straight lines is to follow the straight lines that are built into the fabric. Close enough to straight for me. Just don't pull and stretch the fabric. Then again that will cause other problems. Works great when I am cutting it too, a lot easier than drawing a straight 12' line to follow.
One thing I am playing around with when I get back to sewing is ironing the seams before sewing. I think it makes a cleaner edge and more profesional look. It helps to hold the seam too.
Is that too much to ask? Girls with frikkin' lasers on their heads?
The hanger formly known as "hammock engineer".