When I was trying to sew together a lot of swap blocks I had this very problem. I cut freezer paper the exact size the unfinished squares should be. I ironed the paper to the back of the block. I could then line up the paper instead of the block edge to sew. If the blocks were off by more then 1/4" then I took it apart and re sewed it. If it was cut to small I used the block in the backing.