You have to have an exact 1/4" seam and perfectly cut strips. I had trouble with a block that would not sew up to be the size it should be. I thought I was sewing perfect because I sewed very slow and used die cut strips. I used Wonder Tape and sewed the seam and the block was the right size, no trimming needed. I compared it to the block that wasn't the right size and sure enough I wasn't sewing an accurate 1/4 seam. I was just a stitch width over. IT does make a difference.