When I teach log cabin classes, I encourage squaring up after EACH round (that is, the addition of 4 logs) using the Bias 8 Square ruler from Martingale-That Patchwork Place until the block becomes large enough to use a 12" square. The lines of the Bias 8 are particularly suited to trimming minute amounts from each side of a block to make all blocks the same size at each round.
I do this because the piecing of the traditional log cabin block is in concentric circles.......those who have done any needlepoint embroidery, for instance, realize that when you piece or sew like this, it tends to cause the finished piece (in this case, the block) to slant a bit and pull out of square.
This process makes creating the blocks a bit more time consuming, BUT, having ALL the blocks exactly the same size square, makes setting the blocks together go SO much faster and easier!
Jan in VA