My 4 panniers are packed more for function than front/rear weight (except for a seemingly insane amount of water during a late summer Mojave crossing..I stashed water containers wherever I could fit them). Over the years I've learned that I feel more difference is my bags aren't pretty closely balanced left/right, especially the fronts. I don't actually weigh them, just pick up one in each hand and adjust depending on what I feel.
With that said, I think that the nature of 'functional packing' has put most (not all) of the small/dense stuff in the front (food, kitchen, water) and the bulky/less dense stuff in the rear (sleeping bag, clothes).
Sam