Since you are using sashing between all the blocks, I'd wait until all 3 are finished before "squaring" them up. That way they can all be the same size. Makes it much easier to sew.
Are you losing fabric in your seams?? I;m not sure what type of block you are making. Some of the pieced blocks w/ points need that extra 1/4 inch you are losing so the points don't get lost.
Also, there are several sizes of square shaped rulers you can buy. My favorite is a 12.5 inch one. It just makes life a whole lot easier.
HTH,
Julie