I recently made a sampler quilt for my daughter made of 4-1/2 inch finished blocks, so accuracy was a necessity! I found that my accuracy went way up when I used tiny drops of glue (plain old washable glue in a bottle with a tip that put out very tiny drops; I found a set of three on Amazon). I placed tiny dots of glue at the edges inside the seam allowance, put the two pieces together, pressed with a hot iron to dry quickly, and then allowed to cool before actually sewing them together. I used 50 weight thread in the top and 60 weight thread in the bobbin. I used a scant 1/4-inch or 1-/4 inch seam allowance - whatever got me the finished block size called for.
I wish you well with you sewing!