Patsy's Mom has described one way to sash; you have describe another acceptable method; a third way is to have the side sashing match the height of the blocks, while the sashing between the rows is one long strip that runs the width of the quilt; a fourth ways to sash each block across the top and bottom, then have the vertical sashes run the entire length of the quilt.
my point? there isn't one right or wrong way. whatever is pleasing to your eye and within your skillset is just fine. :P