I switched from using thangles paper piecing or just plain cutting 2 squares RST and cutting them diagonally in half per the formula for making HST's. [I added 1 inch to the size of HST needed so I had to trim up the HST's.]
I now use The Folded Corner Clipper ruler. MY HST'S come out perfect with no trimming. It is good for up to 5".
Eventually I'll experiment with larger rulers and the add a quarter ruler but trim either with this ruler of with the Fons & Porter trimmer ruler.