I find that just one ruler does not do as well as having a few rulers of different length and widths. I also had a plastic's store cut me some clear squares in different sizes so that I can plop one down on top of a block and quickly see if the block is square of where it is off.
My main problem in now getting good cuts is that I don't have the fabric folded correctly. So after every two cuts, I check the fabric to see if it is still square. If not, I do a trimming cut.