I have found thorough trial and error that thread size does affect the seam allowance. So I try to use the same thread throughout the piecing. With "So Fine" from Superior Threads, my seams are more true to 1/4" . With Connecting threads thread, I have to allow almost 1/16" to account for the width of the thread. Finished block size, I like to stay within 1/8" of what it's supposed to be, and I try ( notice I said TRY) to be extremely accurate when star points are right on the seam line. If they aren't quite to the seam line, I can live with it, but if they are chopped off, it really bothers me.