A thinner but strong thread like Aurifil does help when piecing. A consistent seam allowance is as important as the 1/4 inch seam. If you are repeating the same block, as long as they all come out the same size, the pattern will work out with only minor adjustments for sashing or border measurements. The only time I obsess about a block coming out perfectly, is if I am doing an exchange. Guess how I solve that dilemma? I don't do block exchanges. Give yourself a break and you will get better as you gain experience.