I agree, use your rulers. Also use the same brand of rulers for each project. The makers of each brand mark electronically and same brand made to work with same brand----for most accurate cuts. I do not use the lines on mats.
I use my rulers to square cutting from R to L if L hand, or L to R if R handed. Hope this helps. Been teaching for 22 years now and this is how I always teach my new quilters.
Hope all this helps