After you've cut and before you move your ruler, check to make sure
that you made a straight cut. If it's slightly off, now is the time to trim.
For HST, I agree with quilterguy27. Cut slightly bigger then square up.
Check out Lneal's post on squaring up HST's. I bought the ruler and love it.
http://www.quiltingboard.com/t-34271-1.htm