Touring load weight distribution
We can generalize the main areas where one can load weight as such:


Front rack vs. Rear Rack
High (on top of rack) vs. Low (in panniers)


The most commonly accepted points for load distribution are as follows:


Keep dense, heavy items low to the ground. The lower you &amp; your bike's center of gravity is, the more easily you can keep yourself upright. This means loading heavy things into panniers (bottom up, like a grocery bag) and leaving less dense items for the tops of the racks (sleeping bags/pads, etc)
Be careful about putting too much weight on the front wheel. One advantage to having weight on the front wheel is that as you turn, it will turn with you--unlike weight over the back wheel. However, too much weight on the front, especially if the center of gravity is too high, will cause you to over steer, or have to steer very rigidly.
Make sure your load is balanced. You didn't mention anything about this, and although it seems like common sense, I want to make sure it's included. An unbalanced load will cause you to have to compensate by continually steering in the opposite direction. You should be able to roughly balance your pairs of panniers by holding one in each hand.


I haven't used a handlebar bag myself, but the same logic applies. Weight in such a bag will have a very high center of gravity, as well as the ability to turn with you. Therefore, it might be a good place for maps, snacks, phone/keys/wallet, patch kit, etc.