I don't use any particular ruler; I use the marks on my cutting mat to figure out the square and cut accordingly. My mat has a zero line in the middle of the mat and numbers counting outwards from it to the left and to the right, so I can make sure I'm not trimming too much from one side and not enough from the other.
Exception is half square triangles - I use my Bloc Loc rulers to get good corners on those, I butt them against a longer ruler, adjust to proper position, remove the Bloc Loc ruler and cut against the other ruler.