There are times that, no matter how careful you are, one block ends up being slightly larger than its neighbour. You can disguise the problem and sew the two blocks together more easily, by placing the larger block on the bottom. The feed dogs on your sewing machine will help to ease in the extra fullness of the larger block.
I have also gone one step further, when I had a border that just would not behave. I put two lines of gathering stitches on the edge and then treated the seam as if I was attaching a set-in-sleeve to a blouse. It worked very well and was unnoticeable after I removed the gathering stitches and gave it a good pressing.