B.zang -- do you cut standing up or sitting down? I find I get better results standing up, as EasyPeezy said arm length is an issue for me. I also fold the fabric twice so I'm cutting through 4 layers. When I do a number of narrow cuts I check every 3-4 cuts that my fabric is still straight but unfolding a strip and making sure there are no Vs forming at the fold.
Between standing up and losing weight it is no longer such an issue, but when I was larger and sitting my tummy would get into the act as I leaned forward a bit...
I do like textured bottom rulers for my long cuts.