Dips, bumps, wiggles from side to side... all of them add extra limb mass, like Pearlie said. Also, we tend to stress knotty wood less when we tiller, which means those whirlygig areas are naturally overbuilt. I think the mass principle WOULD apply.. as in, it's got too much mass for what you're doing with it, not so much as a principle to follow while building it.
Also, in my experience, I'm more likely to break a snaky bow during tillering. They are harder to read.
Michael