Like you, I was often frustrated with cutting yardage. I used to only buy yardage because it's cheaper per yard than precuts, and then find myself frustrated at the amount of time it would take to press/cut/re-square/repeat only to be dissatisfied with the cuts I was getting because the ruler would slip. I finally invested in a Stripology (Creative Grids) ruler and every time I use it, I love it and am grateful to have it. My only regret is not purchasing it sooner. We're pretty frugal, so I don't normally spend money on gadgets or notions.
The plastic is a little thicker and harder than the June Tailor Shape Cuts, so I prefer the Stripology. You do have to fold the fabric twice (into 4 layers) for the initial cuts. I try to be just as careful in making sure everything is straight and flat, so haven't had a problem with a V at the fold line. And my goodness, the amount of time it saves not having to reposition the ruler after every cut. Good grief. I can cut a jelly roll in less than an hour and it's actually fun! It's so rewarding to use that ruler. It was around $50-55 when I got it on Amazon, but worth every single penny.