I have used a variety of things on the backs of rulers, including the June Tailor Shape Cut, to prevent slipping. The best by far that I have found are the "Grip Strips" from
http://www.guidelines4quilting.com/GripStrips.html . They are inexpensive, easy to apply, and they work! The worst thing I can say is that the website is confusing. The grip strips, though, are wonderful. I am putting them on all of my rulers.
I currently have two grip strips on my June Tailor Shape Cut but plan to experiment with adding 2 more (and maybe even one in the middle) as my attention seems to "slip" occasionally when cutting. I agree with keeping at least one finger off the mat while holding in place.