I am fanatical about everything ending up square. I will sash two opposite sides of blocks and then mark the long piece of sashing between the rows, at every intersection--and I mean every single one. The marking takes a little time and figuring, but I find this method allows me to ease a little or stretch a little where necessary, and makes for a nice square finished product with no wavy edges.
I will keep this in mind for a shadow quilt I am planning. Thanks.