The easiest way for me to square up the HSTs is to have a square ruler the same size as the completed square should be. Center your ruler (with the 90 angle line on your fabric line) and cut around. It's easy to line up again as you turn -- just use the angle line on the square.
My 4 1/2" ruler is just the right size for HSTs that go into a 12" block using 9 squares. Does that make sense?