I remember I was taught in my auto shop class in the early 70's about how the flat tappet lifters are basically rounded on the bottom. Just a little bit so they will rotate on the cams slightly angled lobe. I was also told to put two lifters bottom to bottom and they should rock a tiny bit because they should not be flat. I have never wiped a lobe and I always make sure my lifters rotate just sitting in the lifter bores when I rotate the eng. If any dont I make sure I fix that before assembling the rest of the eng. Ron