Setting up my Great Northern Critter Gitter for the upcoming season and I was having some issues with my arrow flight not being as clean as I hoped. I slipped a piece of a tooth pic to bump the arrow a tiny bit higher and the results were great,
I have Byron Ferguson's "Tuning for Extreme Accuracy", and he goes over this in detail. He seems to be a big believer in it.
Before they started making the shelf with more radius, we put a small piece of wood under our material to raise the arrow for less contact with the shelf.
For years I shot with a Flipper and Berger/Cushion plunger and loved it. Less arrow contact with the shelf the better.