I do find when I have that many HST's to square up the rotating mat does help. I just use my normal 24" ruler though. I don't have many specialty rulers. I have found that using the same ruler - or at least same brand ruler - throughout the quilt makes a difference. I place the 45 degree line on the seam line of my HST and trim from there. Still have some flipping of the ruler but spinning the blocks on the mat does make it easier for me.