I have always squared mine up just like Grammy P--line up the diagonal, trim two sides, rotate, trim the other two sides.
I just bought Eleanor Burn's ruler for squaring up HST and am anxious to use it. Before you press your block open, you line up the seam and trim. Then when you press the block open it is trimmed and the correct size.
This is what it looks like--
http://www.quiltinaday.com/shoponlin...p?i=1087&page=