The width size of ruler I use is determined by the width of the fabric that I am cutting up and the shape of what I am cutting. The wider the ruler the more it anchors the fabric. If I use too wide of a ruler, it tends to "rock" as part of the ruler is not directly on top of the fabric. I have a lof rulers!

Sometimes I do this:
a). position ruler for cut I intend to make.
b). start rotary cutter about center of fabric & cut to top edge
c). start rotary cutter at bottom of fabric and cut to where you began in step b)

I’m making 2 cuts and it seems to work without the fabric or ruler shift I’d get when I got to the top edge using just 1 cut.