Back heat or Belly heat for Arboreal snakes?

[Mikoh4792]

If I were to keep a green tree python, amazon tree boa or even a semi-arboreal snake such as a carpet python in a racking system, would back heat be better? It may seem like the obvious answer since arboreal snakes will spend most of their time off the ground but I've read that people have issues keeping a thermal gradient during extreme cold and hot conditions.(Freezing winter day, Scorching summer day).