I have made one JN quilt (
Agave Garden, the big reveal ). I did not take a class so I read the instructions with the pattern about 3 times until I was sure I understood. It was my understanding from the pattern that the reason for the precutting was to get the least amount of fabric waste and by having the rough shape of the pattern piece already cut out the piecer avoids the pitfall of many PP which is that sometimes those odd angles are difficult to visualize and place so you end up with not enough fabric covering the pattern piece. Additionally I believe she is taking grain into consideration so perhaps her layout of the templates has a purpose when it comes to bias so the complex patterns don't stretch out of shape in critical areas.
I did not cut the way that was illustrated in the pattern. Instead I measured the largest length/width of each unit and cut strips of fabric to that measurement and then I utilized the templates to get the correct angles from my precuts. I did not have any issues with any of the pieces being too small.
I do love her patterns and while on the spendy side, because they include all the foundations, I would not hesitate to do another.