A balanced load on the bike makes for a more stable ride. Most will suggest 40% up front. More is probably better to counterbalance the weight of the rider and the load on the back. OTOH, you can get use to having all the weight on the back if you don't have or need front panniers. I have toured in warm weather with front panniers only and a rack pack. That's makes for a pretty balanced load.

I doubt the gradient of the route has much relativity to load balance. The purpose of balancing the weight distribution is stability.