I have True Grips on most of mine. They are clear, so don't interfere with seeing the fabric or lines on the ruler (because invariably they land where I need to meaure). They do help, but aren't perfect.
Recently saw the Quilter's Sidelock, I think that looks awesome and would try one had I not recently bought the Martelli no slip strip ruler during their redline sale.
For the price we are paying for fabric, combined with how much better patterns come out with precision cutting, having the right tools is worth it!