I use 50wt 2 ply thread or thinner for piecing. One thing is to be very sure there is no veering at the end of each seam. That will cock the edge of the block just enough to be wonky. One thing I did as a beginner was to machine baste the block, press and measure. If it was the correct size then I resewed it. If not exactly the right size, I found where I made the mistake. It takes practice but soon you will have the AhHa moment and you will be piecing like a pro.